What do you think of something like this? It seems recommended by Mozilla. And spoofs usear agent on JS level too (according to developer). I tested, and it indeed do not trigger any Cloudflare spoof detection in opposite to about:config approach.
I mean is it safe to use something similar? Because it really helps (especially on mobile Firefox) to get away from “install our app” pesky notices, if I spoof windows PC (I do not have any windows machine, if it is important)
I would be careful with it as a permanent setup. Changing only the user agent can make you more unusual, not less, because other signals still look like mobile Firefox. For the app-banner problem I’d first try the site’s desktop-mode toggle or Firefox’s built-in request desktop site option. If you do use the add-on, I’d keep it per-site and only for annoying sites, not as a general anti-fingerprinting measure.
Not useful for advanced adversaries.
That depends on your threat model.