Sorry I've missed the 'firefox' part xD.
If that happens next time take a look at htop or some system monitor app and check what uses cpu the most.
Form me honestly it sounds like gnome trackerminer or what was it called. It basically indexes your files so you can search for them fast but it has bad reputation for sudden 100% cpu spikes that seem random. But dont do anything with it until You discover the exact issue.
Maybe You have steam in the background? It sometimes likes to process vulkan shaders in the background, which also might cause rapid cpu spikes
Mind telling how does Krita works for You under xwayland? I am using Krita in flatpak under wayland gnome and Im getting a lot of flickering and gui turning black xD
And firefox has 3%. Its more unpopular to use firefox rather than linux lol
Its not sure how it exactly works, but most probably this captcha processes a lot of data like your mouse movement, mouse click but also your browser fingerprints, search history and ip. You can actually get 'traffic lights' test from this clicking button captacha if You have privacy protection in place, such as using brave, tor, firefox or mullvad browser and/or vpn, pluse some privacy browser extensions
From every linux related ltt video or wan show I've seen. My personal favourite is probably a video about AtlasOS, where he presents modified windows os with many security vulnerabilities and makes some comment like "who needs linux when you have this!" (not actual citation, dont remeber exact wording, feel free to check it out)
Linus is just a windows fanboy, just like i am a linux fanboy
The reason is most likely TF2 not playing nicely with wayland. I have nvidia and I needed to add this command "SDL_VIDEODRIVER=x11 %command%" To be able to run it. If You can check if game runs on x11
If You do not feel comfortable to leave Windows for gaming then of course keep it and dual boot PC with Your distro. Also, You can go to protondb to check how Your games run on linux