It is intentional that pressing the ESC key makes you leave Full Screen Mode.

What reasons/issues do you have that makes you press ESC so often ?

Well, closing any popups on websites & search panels and stuff. Generally, on previous versions ESC hadn't been disrupting fullscreen (in, like, 3+ years I'm using FF as my daily driver), so can't tell whether this change is for good — I don't have or want access to your product metrics and such — but it's kinda a deal-breaker for me. Having to be mindful of whether the focus is on any particular site element doesn't sound like a good thing to spend small bits of attention I still do wield.

Anyway, I suppose your question-instead-of-answer means there's no way to switch this off, so the options I do have are 1) building my own extension catching keydowns or something on window-level; 2) switching to, I don't know, Chrome. Both are, to say the least, sub-optimal, and that's why I asked on whether that was an intentional change.

Ok, the real answer for my q is: "Switch browser.fullscreen.exit_on_escape in about:config to `false`"

I wasn't aware that this browser.fullscreen.exit_on_escape pref exists. Leaving full screen mode via ESC appears to be a Mac feature added in 92 (1688584) and was disabled for 93 by adding the browser.fullscreen.exit_on_escape pref (1726810;false) and got enabled again in 94 (1732367;true).

A very annoying change of defaults. Thank you for the solution, this made Firefox almost unusable.

I just created an account for the first time to vote up the solution and vote down @cor-el's response.

This the thread is the first result I got when searching for a solution to the problem I was having with MacOS fullscreen mode and escape-key behavior changes.

Thank you @kadmil for asking and answering this question, the behavior change was very annoying and unexpected.

I returned to Firefox from Chrome and have been very happy, up until this behavior change ruined my constant use of fullscreen mode on a tiny macbook air 11.

This is a legitimate issue and question, asked honestly and well, with a bit of humor. It deserved a better community response.

Poor moderator responses like this give Firefox a bad name. Please do better.