I noticed my Firefox browser has been taking longer and longer to start up (Vivaldi was near-instant) and I noticed its cache was well over 1 GB in size. Clearing the cache worked but you really need to have it do this automatically. The guide below shows you how to do it but be careful to only have "cache" ticked otherwise it will also delete your site cookies, passwords, login info, etc.
The Firefox cache temporarily stores parts of websites you visit. Learn how to clear it to fix issues with how websites work or look.