Maybe I’ve installed too many plugins? I have close to 20.
Edit: It was just an issue on Arch Linux.
No, I am 40 plugins all active and they have not changed my browser load time almost at all.
Make sure there are no other issues. If you are on Arch Linux, there is a problem with xdg-desktop-portal-gnome that if installed will slowdown loading of many programs.
As others have said, Librewolf and Waterfox are both forks of Firefox.
That said, i am using Firefox on Fedora from the repos with no issues. Do you use the snap version? Ubuntu ships it default and it is as trash as snaps are in general (it has exactly the problem you stated, taking too long to open). There are guides to install the .deb version (apt installs the snap version…) or you could use flatpak. Another thing to check is if you have a new addon / plugin running thats hogging resources.
Another thing you can try is some kind of caching solution, like profile-sync-daemon that will out your profile in RAM.