How do I get rid of the Ubuntu scrollbars and theming in Firefox 46.0?
Solution 1:
This appears to be a bug with the GTK3 theme, probably Firefox is using the wrong theme information for the pop-ups and the scroll bar.
I partially fixed this for myself by switching the theme for just Firefox:
GTK_THEME=radiance firefox
You can add this "permanently" by editing the launcher file:
sudo gedit /usr/bin/firefox
and adding the line (after #!/bin/sh):
export GTK_THEME=radiance
It'd probably be a good idea to file a bug against this, but now that I've finally gotten rid of the awful dark menus I'm less motivated to find the root cause of the problem ;)
Solution 2:
What you could do is to install and use a complete theme for Firefox that includes its own scrollbars etc. This won't look like the old default, but I bet you can find something similar or even nicer.
In the Tools menu, select Add-ons, there go to the Get add-ons tab and scroll down the page. In the lower left corner is a link See all complete themes. This should bring you to Mozilla's Firefox homepage and their complete-theme site. Alternatively, click this link.
Pick one of the available free themes that is compatible with Firefox 46 and pleases your eyes, click on its green + Add to Firefox button and after the download completed, confirm the speech bubble asking for your permission by clicking Install.
Then you will be asked to restart Firefox which you should do, and the new theme will get enabled.
To change the theme again to a different or the default one and to remove no longer wanted themes, open the Tools menu, select Add-ons and navigate to the Appearance tab where you see a list of all installed themes.
PS: My personal favourites are "BlackFox V2" and "NASA Night Launch" (Yes, I love dark themes...)