Firefox and Chrome are both memory hogs. They eat up more and more memory the longer they’re open.
I suggest finding the setting that saves the tabs you’re using. Then close them once in a while and re-open them when things get slow.
Sometimes I use Control-Shift-D (in Firefox… actually I use Waterfox, but same) to bookmark all open tabs, then close it, making sure Download manager is also closed. Then I wait a few minutes for my computer to “get happy” again, then open it up again.
I also keep Opera on hand as well. It’s VERY good with Youtube, although not as good with some other sites, and I’ll sometimes switch over to it from time to time.
Also, I suggest “NoScript” add-on for Firefox and get used to using it. That way you pick and choose what Scripts are running from sites. It takes a bit to figure out what scripts are really necessary for a site to function, and sometimes you have to refresh a page, but getting a sense of control over the pages is worth it. Plus it’s kinda cool seeing what’s happening behind the scenes a little.
It allows you to keep from getting ads on some pages, which means no unnecessary pop up videos suddenly playing and stuff.