Warning: The proposed workaround in this blog post might be rendered unnecessary in the future.
Out of the box, Firefox with client-side decorations doesn’t look good because captions in inactive tabs blend with the background.
Fortunately, this issue can be easily fixed by adding a style rule in userChrome.css file. First of all, you need to enable the userChrome.css functionality if you run Firefox 69 or later. Go to about:config page, and set toolkit.legacyUserProfileCustomizations.stylesheets to true.
Next, open the profile directory and create userChrome.css file in a sub-directory named “chrome.” If there is no “chrome” sub-directory, create one. In case you don’t know where the profile directory is, open about:support and click the “Open Directory” button next to “Profile Directory.”
In userChrome.css file, add the following rule
:root[tabsintitlebar] #TabsToolbar:not(:-moz-lwtheme):not(:-moz-window-inactive) {
color: #eff0f1 !important;
}
and restart Firefox.
Closing words
Even though the userChrome.css functionality hasn’t been officially deprecated, it’s still better to avoid using it, but as a short-term solution, it’s good enough. Ideally, this minor issue should be fixed upstream so everything “just works” out of the box and no hacks are needed.
Vlad Zahorodnii's Blog 