advice on browser change

It's really hard to compare any two computers.

My Firefox profiles all include about a dozen add-ons. When I am working on a research/educational project, I may have 10 or 12 tabs open, plus MS Office (Word and PPT), plus SnagIt, Photoshop, Mailwasher Pro and sometimes Pandora or other radio streaming service.
I see no performance issues or slowness from Firefox. But I have pretty robust hardware.
And, after a hour or two, I often close the browser session and clear cache/cookies/history/data when I take my "stretch break". I then start a new session.

Most browsers, including Firefox, will take and use available memory to enhance one's experience. That is normal.
But, if one is running many other, resource-intensive applications and/or if one's hardware specs are less than robust, I suppose a slow-down is possible.
Another aspect to check: real-time security applications and global plug-ins. They, too, can cause a performance "hit".

Even so, from time to time, it's often a good idea to start with a clean profile (one of the steps suggested in the tutorial I mentioned earlier). That usually fixes minor performance problems.

The bottom line: while it's easy to lay all the blame on the software itself, many performance issues originate at least in part with the user's profile, system specs or other factors.

Cheers,
MM

I'd recommend SlimJet or Pale Moon, two lesser-known browsers, the former is based on Chromium engine and has lots of features, while the latter is based on Firefox. If you have a lower-end PC, I'd recommend Pale Moon.