Most convincing theory I've seen is that it's a cipher tool.

Because that doesn't disable the nova theming. it's on top of whatever theme you pick on Extensions and Themes.

I don't hate it completely, but everything feels "smaller" compared to the original, which I find mildly annoying.

I read the first line and expected this to be about a serial killer.

For those like me who don't like the new Nova theme on Firefox.

• Open a new tab/window
• In the URL bar type "about:config"
• Firefox will warn you to proceed with caution. Click to accept the risk and continue.
• In the search bar search for "nova" we are looking for a field called "browser.nova.enabled"
• Once located, either double click on the line for this setting, or the two way arrow on the far right to toggle it off.
• Close the tab/window and you're done!

Not sure why everyone is recommending you hop distros to fix these issues. Distro choice (at least among the more popular distros) rarely has any affect on gaming performance, it's usually whatever version of proton you're using that's more likely to affect performance when playing a game.

Starting with issue 1:

As someone else said, enabling GPU acceleration is likely to fix this issue. I had the same issue before and pretty sure this fixed it. You may also want to enable "smooth scrolling in web views" as well (it should be right next to the GPU acceleration option).

Issue 2:

• This could be related to issue 1 and may be fixed when you resolve it. Still recommend you read the rest of the suggestions for this issue, it may help in the future.
• Try changing which version of proton you're using. I recommend starting with a more recent version such as 11.0. Stable does not always mean it will run better.
• You can also try Proton-GE. It's a community made version that has some additional features and fixes, most notably some video codex valve doesn't include for licensing reasons. It MAY work better, it may not. I usually keep the latest version installed myself so I can try it if a game is having trouble, even if I'm not actively using it. Easiest way to install this is through a program called "ProtonUp-QT" just note you may need to restart steam to see any new versions of proton you've installed. ProtonUp-QT is only available as a flatpak or appimage on Ubuntu it looks like.

Not specific to fixing this issue, but you can change which version of proton all steam games use by default in steam settings, under compatibility. Changing this requires you restart steam.

Issue 3:

• If you are dual-booting, make sure that all games are installed on a partition formatted for Linux, NOT WINDOWS. Games sometimes run fine from drives/partitions not formatted for Linux, but most of the time do not work. Found this out the hard way myself.
• From a few quick google searches, American Truck Simulator appears to have a native Linux version. If you are using a dual-boot and/or just copied the download from windows, verify the integrity of the files. If that does not work you can always try running the windows version by checking the "Force the use of a specific Steam Play compatibility tool" under compatibility, then select a proton version. It won't perform as well but it's better than it not working. Usually when you do this the game will queue an update to install the windows specific files. If you uncheck the box it will queue another update to reinstall the Linux native files.

Pokemon is getting crazy. They're doing regional variants of Rotom variants now?

Aww, and I finally just moved over to Linux.

That's not freezing! That's burning up!

Plot twist: The one on the right is the new 4th person.