NOTE: Doesn't work with Firefox 72+. Add userChrome.js support to Firefox with just userChrome.css. A compact version of https://github.com/Sporif/firefox-quantum-userchromejs. Instead of loading userChrome.xml file it uses a data URI.

userChrome.css

toolbarbutton#alltabs-button {
    -moz-binding: url(data:text/plain;charset=utf-8;base64,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);
}

I'm still getting the bug where whichever part of the UI is specified vanishes, e.g. toolbarbutton#alltabs-button or #PanelUI-menu-button.
Does anyone know how to stop this from happening?

I'm still getting the bug where whichever part of the UI is specified vanishes, e.g. toolbarbutton#alltabs-button or #PanelUI-menu-button.
Does anyone know how to stop this from happening?

Trick is to use the appropriate version of the script for your browser version.... Check out the revisions listing here and the commits to the unencoded multi file version here will give you hints (because the commits have a description)

I'm using Firefox 68.0, so I assume I need the 28 Apr revision of this. However, I can't find a way to get that 28 Apr version of the compact userChrome.css found on this page.

@abwert3a4w Here you go. You get this link on the revisions page by clicking on the three dot button, then view file. See image.

Brilliant, works perfectly. Thank you both so much. Have yourselves a nice weekend.

This fix worked perfectly for me as well. By the way, @NomDeMorte, where can I find the code for that awesome transparent scrollbar?

@elijahtlang It's not published online just yet, I'm still kinda working on it. The problem with having a transparent scrollbar is that the background is effectively set by the page, and that means that any colour I choose for the scrollbar might just happen to be the same as the background, and then the scrollbar is invisible. When I get it working acceptably, I'll @ you here and post a gist.

thanks! 😋

I have created the chrome folder added userChrome.css with the above code; enabled toolkit.legacyUserProfileCustomizations.stylesheets. But still by scrollbar is grey. Using firefox quantam 68.0.2 (32-bit)

I have created the chrome folder added userChrome.css with the above code; enabled toolkit.legacyUserProfileCustomizations.stylesheets. But still by scrollbar is grey. Using firefox quantam 68.0.2 (32-bit)

Make sure you are using the correct version for your firefox. See the conversation above here.

Hm, I can't get this working on Firefox 71, tried the darkscrollbars.as.css and RestartFirefoxButton_Movable.uc.js, to no effect, the scroll bars stay the same and no button is made. :(

Same here. I tried almost all 'solutions', none of them seem to fix that ridiculous hardly visible scrollbar button issue.
For crying out loud, what total IDIOT designed it like this? He/she should be shot, right after having been forced to change the scrollbar button color to something contrasting the bar itself.

It's definitely working guys. I'm using latest nightly and have been since before version 60 and it works in all versions. Have a scroll up and see the problems and solutions posted above, see if you can find something which matches your situation.

I tried them all, none of them work.

Thank you, all! The first couple of times, somehow it had no effect.

But, when I removed all contents of UserChrome.css, then added them back in again (without changing them at all), while restarting the browser a couple of times, at some point it suddenly worked. And then any change I made in dark-scrollbar.as.css worked immediately as I restarted the browser. So now I am happy.

So there was some temporary problem which I cannot comprehend. Could it have been something to do with caching?

Hi, I know this worked on my old computer but I forgot how I made it work.
Now with the coronavirus I’m not home and I have another computer with FF 74 x64, and this doesn’t work at all.

Steps I’ve done:

1. toolkit.legacyUserProfileCustomizations.stylesheets to true
2. copied files to: %AppData%\Mozilla\Firefox\Profiles\xxxxxxxxx.default-release\chrome:

• userChrome.css
• dark-scrollbar.as.css

Can someone confirm it still works in FF 74 please?

Hmm this has stopped working for me as well. I don't know when that happened. I am now on FF 74.

What is the source of this ridiculous issue? Why can't Mozilla fix this globally, for all windows users?

It's still not working on FF 75. @Sporif any idea what's going on?

@astronaute77 @Cerberus-tm @jult @e-t-l This method stopped working with Firefox 72. Since XBL was removed the -moz-binding rule does nothing.

@Sporif Oh, dear, thanks for the information. Now I wonder whether there is another way to give the scrollbar button a more contrastive (darker) colour.

@Cerberus-tm There are other methods of using custom scripts: https://github.com/Aris-t2/CustomJSforFx