This is a nifty idea. I do something similar, not by domain, but by the tab's label, by editing my userChrome.css file. For example, if one wanted to make Hacker News tab lines orange, but only when focused:
Thanks for your feedback. I didnt even know about the userChrome.css file. It's cool that FF allows the user all that customization. My original plan was to build this extension for Google Chrome, as that was my main browser for development, however it's not actually possible because they don't allow dynamic themes.
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[ 4.1 ms ] story [ 12.7 ms ] threadtab.tabbrowser-tab[label$="Hacker News"] .tab-line[selected="true"] { background-color: #FF6600 !important; }
Or if one wanted them to be orange all the time:
tab.tabbrowser-tab[label$="Hacker News"] .tab-line { background-color: #FF6600 !important; }