my_firefox_theme VS FirefoxCSS-Store

Or you can try themes from: https://firefoxcss-store.github.io/ some of them integrate well with Tree Style Tab, but you can use Sidebery if you like that one. My personal preference is My Firefox Theme — screenshots

Try: https://github.com/not-holar/my_firefox_theme (works with Tree Style Tabs)

search for it on https://firefoxcss-store.github.io/ if you need assistance go to r/FirefoxCSS

yes, we can put "links of interest" on the web to collect all the communities, websites, or whatever related to firefox themes.

You can also remove that unnecessarily large sidebar header that Firefox insists on not letting users disable via normal means with userchrome mods. Some such setups can be seen at https://firefoxcss-store.github.io/.

firefox's ui is very customizable with css and i dont see why flatpak would change that. here are some examples https://firefoxcss-store.github.io/

I like to use this

If somehow this could be integrated with the css store down the line, I can see a lot more casual Firefox users finally trying out css customisations.

you can find others here https://firefoxcss-store.github.io/

You might want to take a look at Firefox CSS. There are some themes for home page and at the end of the site is guide: https://firefoxcss-store.github.io/