Restore classic Photon look on Firefox 91+

Enable userChrome.css by setting toolkit.legacyUserProfileCustomizations.stylesheets preference value to true in about:config.
Copy the chrome folder into your profile folder.
Change the browser.tabs.tabMinWidth preference value to 76, which is the default value on Photon.

Screenshots

Firefox 78 ESR:

Firefox 91 with this userChrome.css:

This configuration contains a few non-Photon changes which I find convenient:

@import "tab_line_loading_indicator.css";
Disable the tab label mask, which makes the text label on the tab button hard to read.
Proton's enlarged close button on tabs are kept.
Media mute button on tabs does not replace the tab favicon, but it's not in the Photon style, which puts the mute button next to the left of close button. Because Proton changed the HTML structure of the mute button, it would be difficult to put the mute button back to the Photon position, so instead I moved the mute button to become "superscripted" to the favicon, similar to how Proton handle mute buttons on pinned tabs.

This tweak is not perfect. I focused on undoing the most annoying change introduced in Proton:

The larger meaningless margin on the tab bar and navigation bar.
"Tabs" don't look like tabs anymore. They should be connected to the parts below.
tabMinWidth is so small that only the first character of the tab title is visible when you have many tabs open. See screenshot.

There are many other parts changed by Proton, but these are less annoying in my opinion. If I have time, I will also try to revert them as well.

This tweak has only been tested on macOS 11.5.1 so far. I will improve it for KDE later.

Most of the CSS files come from https://github.com/MrOtherGuy/firefox-csshacks , but I tweaked it further.

Mozilla Public License 2.0