Tzeentch

Are you using Firefox within a flatpak perchance ?

There seems to be a bug to it relating to use of bitmap fonts, you can fix the issue by disabling them via a config file in firefox's fonts: https://bugzilla.mozilla.org/show_bug.cgi?id=1621915

There's a few options

Newpipe, uses it's own extractor, also can act as a player for SoundCloud and Peertube, there's also a fork with SponsorBlock: https://newpipe.net/

Libretube, uses the Piped api, generally seen as the one with the best UI: https://github.com/libre-tube/LibreTube

Clipious, uses the Invidious api: https://github.com/lamarios/clipious

Could also watch videos in a browser from an Invidious or Piped instance I guess

And while I'm at it I'll also mention there's a client option on desktop too, Freetube: https://freetubeapp.io/

PDF4QT, bit of a newcomer, but its a free and foss pdf editor with the capabilities of propriatery equivalents: https://jakubmelka.github.io/

The themes section on the desktop integration doc here might be of use: https://docs.flatpak.org/en/latest/desktop-integration.html

This may be useful for further elaboration: https://www.reddit.com/r/flatpak/comments/y9jmqj/the_general_flatpak_qt_and_gtk_theming_guide/

Going off of these guides my best guess would be that you don't yet have kvantum or some other theme runtime installed within flatpak

I'll also mention Stylepack exists for automating and streamlining theme installation, it's only for GTK currently however but maybe will be of use in future: https://github.com/refi64/stylepak

Two utilities that may be handy for you here:

Pakrat: Automates and simplifies the process of creating alliases for flatpaks, good if you just need to make a few programs be simplified

Fuzzpak: Lets you do fuzzy searches for flatpaks(as in you just write fuzzpak inkscape and it auto looks for something with inkscape in the flatpak folder and launches it), good for when you want to simplify launching flatpaks in general without doing the process of configuring stuff manually

mokutil --disable-validation

Though this sounds more like a case of needing to disable secure boot in your bios ? As far as i know Mint has no secure boot support, and the mokutil keys of a previous installation wouldn't affect a fresh one

You can, infact there's outright a mesa git runtime one can add, i don't imagine too many systems roll so fast as to outpace it https://docs.flatpak.org/en/latest/available-runtimes.html

No, that's a different protocol, the tearing patches, already merged, but i believe we're still waiting on some patches to the kernel to get them fully working(i think it was kernel 6.8 that had them), Nvidia's drivers also need to add support for it, they've given no timeline for that so far however

https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/65

That blogpost is considered to be somewhat flawed with its information, as explained here: https://tesk.page/2021/02/11/response-to-flatkill-org/