If you haven't read that 1416044 Bug report "Add telemetry for userChrome.css usage" yet, please do so. But whatever you do please don't post any comments, which might cause that report to get hidden from view for all but the developers involved.BobbyPhoenix wrote:"userChrome.css may be next on the chopping block" Just giving my 2 cents. I don't think userChrome (and userContent for that matter) will go away. Even if people are breaking things with them.
A Telemetry probe for the presence of a userChrome.css file was added to for 58.0b11 and once they see how many users have that file they may start grabbing the files size or maybe start looking into what parts of Firefox it affects by "scraping" some of the larger files they stumble across. From there they could start blocking specific types of CSS code if they don't like what they see. From there it is a very small step to just eliminating that option altogether; not as a doomsday prophesy but as a continuation of what Mozilla has already done in the last 6 or 7 years.
I am blocking Telemetry and hope other users do too, to throw a wrench into their plans to snoop inside my Firefox Profile. But that may turn out to be us shooting our self in the foot - IOW backfire on us due to a lack of "real world" data from experienced users who can handle working with userChrome.css and to not create "user induced" problems.
My overall "trust" of Mozilla to do the right thing for its long term, more experienced users disappeared a long time ago either with Firefox 4.0 or Firefox 29; and if not then it, disappeared with the signing "feature" and how poorly that was rolled out. The latest version that I use everyday is Firefox 47.0.1 while my most used version is ESR 38.7.1 .