You should avoid the stock Firefox install on Steam Deck as it's badly outdated (updated)
For all the pros of the Steam Deck like the full KDE Plasma desktop mode, there's a bit of a sore spot with Valve being too slow to update some of the included software. ARTICLE UPDATED.
How to setup OpenMW for modern Morrowind on Linux / SteamOS and Steam Deck
How to install Hollow Knight: Silksong mods on Linux, SteamOS and Steam Deck