(In reply to Evgeny from comment #1)
Could you please clarify when you plan to remove
browser.proton.enabled which allows to enable (more or less) old style of the browser?
The pref will likely be turned into a no-op during the Firefox 90 cycle.
Could you please consider keeping this option at least in the Firefox 91 ESR?
No, the overhead both in terms of code and in terms of complexity, is significant - so much so that I'm going to split this bug up into a few different dependencies, with a rough schedule as to when each of these is happening, with the goal of making uplifts in the 90 cycle straightforward (instead of having to rebase around conditionals that are there in one branch and not another).
If there are concrete issues you have with the new styling, please file bugs on those insofar as they are not on file already.
Points: 2 → ---
Component: Theme → General
OS: Unspecified → All
Hardware: Unspecified → Desktop
Summary: Remove non-Proton branches from our CSS and JS, and Proton prefs → [meta] Remove non-Proton branches from our CSS and JS, and Proton prefs