[Ayesh]

I'm somewhat active on r/FirefoxCSS, and have substantially customized my Firefox UI. Never saw any deprecation notices in the console, or a message on the subreddit.

I highly doubt Firefox would ever drop support for userChrome. The themes aren't nearly as powerful and low level. The about:config toggles works as a footgun protection. I have yet to see any technical reason for them to drop support.

That said, Firefox has dropped support for small yet useful features for no apparent reason, so there is a non-zero chance of them dropping support for userChrome customizations.

[rurp]

Killing userChrome would be extremely on brand for Mozilla, so I'm sure it will happen at some point. Killing features beloved by a subset of power users appears to be a guiding design principle there. I've seen userChrome suggested as the fix for all kinds of UX regressions that Firefox pushed on users. But if Firefox is willing to go out of its way to break the UX in various ways, why would I ever trust it to maintain that support?

[userbinator]

Of course people are going to find ways around it, simply because the alternatives are still so much worse, but it definitely disappoints me to see that we have to be fighting this constant war with the developers at Mozilla who are continuing to oppress users while thinking that they're doing the right thing.

[novok]

I think it will happen mainly because it's a feature that is behind about:config.