i also use arch and firefox/mullvad browser do take a lot longer to launch than normal. my problem could be some dependency missing (ive already got problems with video codecs i have no idea how to fix) or a broken swap partition. So, does reinstalling firefox help? and do you have a swap part? if yes, how big did yoy make it?

There’s a few suggestions about debugging it in Fedora, you should be able to replicate most of them. https://fedoraproject.org/wiki/How_to_debug_Firefox_problems

What’s your DE?

Me too, never slow. Is this machine really old?