That's not really the car with Firefox in my experience that is an issue that's common to chrome (and as such electron etc...). I haven't refreshed my Firefox in probably 5+ years and it's very snappy despite me having a lot of extensions installed.

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.

I have used FF for many many years and the way to keep it fast is, on exit, to delete all data.

Can someone elaborate on what it means to refresh a browser in this context?

Update it? Clear cache?