userChromeJS

Tested on Firefox Developer Edition 131.0b4.

Instructions

Video running installation steps below: https://youtu.be/_4fdUdp3G4o

Download this zip file and extract its content to Firefox installation folder (usually C:\Program Files\Mozilla Firefox). For Linux paths, read this. And this for macOS paths.
Click Firefox menu button (☰) -> Help -> More troubleshooting information or simply open about:support. Find "Profile Folder", then click "Open folder". In there, create a new folder called chrome.
Download one of the files below and extract its content in chrome folder.

Now, if you're only interested in extensions, you can skip to step 6.

Save the desired userChromeJS scripts into chrome. Read below the description of some of them.
If you want a button to manage your scripts, including the ability to disable/enable scripts without needing to restart¹ Firefox or Thunderbird, save rebuild_userChrome.uc.js into chrome.
Open about:support and click "Clear startup cache…" to force Firefox to load userChromeJS on the next startup.
Restart Firefox.

¹: Not all scripts are restartless. These have @shutdown at the beginning of the code. Almost all scripts on this page were written by me to be restartless, but almost all scripts you get from other sources are not.

userChromeJS scripts

ᵀᴮ: also compatible with Thunderbird

(click to expand)

Besides that, no more than one tab should play audio at the same time. Each tab paused by another tab that starts playing is added to a stack. So if I open a new YouTube video while there's already one playing, the new tab starts playing and the other is paused. When the video ends or when I pause it, the first YouTube tab resumes playing.

As for now, I only added support for a few sites, like Deezer, Spotify and YouTube. I chose not to support next/previous in YouTube, only play/pause.

Download link.

Have you ever been frustrated because you wanted Firefox to autofill https://www.youtube.com/feed/subscriptions when you type "youtube" in location bar? That's because Firefox only autofills domains, so it doesn't go beyond https://www.youtube.com/. Also because of this, Firefox will never autofill https://www.reddit.com/r/firefox/ when you type firefox, no matter how many times you've visited that page. A common case is new Firefox users complaining that browser isn't autocompleting gmail.com after typing gm.

I don't like this. Firefox should be smart to always prioritize the page with higher visit score in browser history. That's what this script fixes. It preselects the first suggestion from address bar. For instance, if this page is the first suggestion when you type xiaoxiaoflood, you don't need to press down arrow key before Enter.

Pages you often visit always rise to the first position, so accessing any frequent page will be as easy as typing no more than three chars + Enter, like just you + Enter to load YouTube Feed directly instead of YouTube homepage. It's even possible to teach Firefox to load YouTube Feed with y and YouTube Homepage with yo, it's just a matter of practice.

This script replaces urlbar autocomplete, so browser.urlbar.autoFill is disabled on install. If at any time you miss domain autofill, you still sort of can achieve that by pressing Tab IF the domain of first suggestion matches what you've typed so far. Example: you typed git and the first suggestion is from github.com. Pressing Tab key will autocomplete the domain even if the first suggestion is not just github.com - it may be github.com/whatever. But if the typed input doesn't match the domain of the first suggestion, then Tab key will have default behavior of selecting next suggestion just like down arrow key.

By default, first entry is only selected if typed input is a single word. That's because when I type multiple words I'm probably wanting to make a search with my default search engine, not to load a URL from history. You can change that if you want: find AUTOSELECT_WITH_SPACE at the beginning of the code of this script, replace false by true, save then reload.

Download link.

Screenshot:

Download link.

Disables autoinstall updates for every addon.
Just after the daily check, if there's an addon update available it will open Add-ons Manager directly in "Available Updates" view, so that you can track changes before updating (you can click on the extension, then click "Release Notes" button).
You can fill ignoreList if you are deliberately using an outdated version of an extension and don't want to be notified that an update is available.

Download link.

You need to set a master password in Firefox Options > Privacy & Security > [×] Use a Primary Password.

Download link.

Locked:

Unlocked:

Left-click to minimize (so I can't close it accidentally).
Right-click to close.
Middleclick restores to fixed position/size (edit script code with your preferred values). If you want to restore to previous position/size, use Shift + Middleclick.

Download link.

Hold right-click, then roll mouse wheel up or down to switch to next/previous tab.
Hold right-click, move down and release to scroll to bottom of the page (or move up to go the top). If the cursor was over an image, moving down will open a new tab for reverse image Google search instead of scrolling to bottom.
Hold right-click, then left-click to switch to the last selected tab.
Hold left-click, then right-click to reload current tab.
Hold right-click, then middle-click (wheel button) to close current tab.
Hold right-click, move left and release to copy URL of link or media under the cursor.
Hold right-click, move right and release to open a new tab with the link or media under the cursor.
Hold middle-click, move left and release to copy selected text or image under the cursor.
Hold middle-click, move right and release to paste.
Hold left-click, then press forward button (for mouse with extra buttons) to switch to next group/panel (specific compatibility with Sidebery extension).

Gestures are cumulative if possible, so holding right-click + turning mouse wheel will activate a different tab for each "tick" in the wheel.

Advantages over existing extensions:

Extensions can't run over Firefox interface, so gestures don't work over Firefox toolbars.
Extensions can't run on privileged pages like about:config and view-source.
Extensions need the page to start loading to run, causing gestures to fail sometimes.
Extensions can't set gestures for 4th and 5th mouse buttons (Back and Forward).
Unlike extensions that are limited by existing APIs, userChromeJS has unrestricted access to Firefox internals, so you can do almost whatever you want if you write code for that.

Download link - extract it in chrome folder.

When Firefox introduced containers, I created this script to get some features that I missed from Multifox, the legacy addon that implemented "containers" years before Firefox having this feature by default. Since then, Firefox has added some things this script had, so I removed them. But I still use it for two things:

New tabs (Ctrl+T or New Tab button) inherit the container of current tab (except for Private Tabs).
The label in urlbar serves as menubutton to reopen current tab in other container. With left click, current tab is replaced. With middleclick, a new tab opens without closing the other one.

Download link.

The main difference between this and extensions like Redirector it that these Firefox extensions record both pre-redirect and final URLs in history. I want it to record just the final URL.

This script can also do more complex things like running a JS function with regex results.

Finally, the main reason why I wrote this was to integrate it with Link Status Redux. When I point the mouse to a link that I've already visited, LSR displays the time of last visit. This is extremely useful for me to know if I have already visited the page and to track changes since last visit.

LSR uses Redirector rules to replace links directly in page (Redirector extension doesn't do this, it redirects only when you try to load the URL). Many URLs have gibberish at the end, I have rules to remove it, then the URL remains clean and LST can track last visit correctly (because the gibberish is different every time, generating different URLs for what is actually the same page).

Note: the list of rules in the script is just an example, mine is much bigger.

Download link.

Screenshots:

Download link.

Screenshots (yes, I'm using the old Stylish icon):

Download link - extract it in chrome folder.