I know EXACTLY what you mean - I also use tons of browsers and tabs, especially when I'm researching. Right now, we're planning a trip to SE Asia and I've got just tons of windows and tabs open for different cities, countries, activities). Firefox 64 just couldn't keep up.
So, I recently changed from Firefox 64 to Waterfox, using 64 bit windows 7 pro (quad core @ 2.5). With Firefox (64) I would only be able to open "so many" browsers/tabs before things started to slow down - around 4.5 GB memory. Right now, I'm running FAR more tabs/windows with Waterfox, am using 7.45 GB memory (out of 8 total) and things are still nearly as smooth as when using only 3-4 GB total.
THANK YOU, Waterfox!