Fortress Forever is finally playable fully on Linux using Proton-GE version 7-41, although there's a bug where if you change the settings then try to connect to a server: the game crashes. This issue, however, is very easily solved by rebooting the game.
If you need a quick rundown on how to download and install Proton-GE, I've made a video about that already. It is technically about a different game, but the problem is still similar enough to apply pretty much the same steps.
It plays pretty well, the only problem is when a local server changes maps, and the font for some of the menus(and the chat) being very... bad looking. It's not the game's fault for the second one, however, it is still something that's annoying if you don't want to do too much tinkering(I was never able to fix it, though).
I'm glad to be able to play this great game on my (used to be) daily drive OS without having to open Bottles every time I want to play Fortress Forever.
Whenever I'm playing Fortress Forever, I'm usually on MaxTF's Midwest server. The reason why is that it's the server that I have the lowest ping on, still not great ping though.