Yeah, Firefox has some bogging and freezing issues but most of them have a solution through the Configuration settings with "About:Config" typed on the Address bar.. those settings can be enabled, disabled or their data values changed.
To find a solution for a specific FF issue, just type a short description of it on a search engine, and you may find something. This one for example... it has lately been freezing my installed FF and I had put off solving it... I had solved it before, some time ago, but I had forgot how to... it took me less than a couple minutes to refind it... and here it is.
How to Stop Script Errors on Firefox
http://smallbusiness.chron.com/stop-script-errors-firefox-48954.html
Other Firefox script error solutions.
https://www.google.com/?hl=en&gws_rd=ssl#hl=en&q=which+about:config+to+disable+scripting+errors+that+block+firefox
DDR 2100 RAM should be ok.. the speed only makes it a bit slower than 2700/3200 but the total physical memory size is also important, probably more.
There are some light Linux distros that would surely run much faster than Windows XP on your system resources. I have tried Vector Linux 6.0 and UIbuntu 9.04 on the old Dell and these two were much faster than XP... Vector a bit faster than Ubuntu 9.04... the later editions of Ubuntu are not so light anymore but 9.04 is even faster than 7.04.
Linux has a few lighter distros like Puppy Linux which is probably the faster one.. but you can try others searchig for "Light or Lightest Linux Distros"
6 Lightweight Linux Distributions
http://www.makeuseof.com/tag/6-lightweight-linux-distributions-give-pc-lease-life/