I have not experienced a noticable slow down over the last ten years of using Firefox, even on modest hardware from from the early 2010's. That said, this year

I have not experienced a noticable slow down over the last ten years of using Firefox, even on modest hardware from from the early 2010's. That said, this year I enabled clearing the browsing history and cache every time I close Firefox, which keeps my address bar clutter free and hyper responsive. Searching trough my bookmarks is instant! It keeps the browser lean and clean.