>>>Is it because we are still on Windows XP?
Well I'm using Firefox with XP and it's working fine for me!
>>>I followed Chris's instructions
Apparently not the bit where I wrote:
"If you do decide to reset Firefox, go to Tools > Extensions > Add-ons first, and note down the names of any extensions that you want to keep, as you'll need to reinstall them after resetting"!
It's possible that you didn't have any extensions installed anyway, Ann. The ones I've got include the British English Dictionary. (Type 'color colour' into an AB answer box to check whether you're using British English or US English. 'color' should be underlined as incorrect. If 'colour' is underlined you'll need to install the British English Dictionary - instructions below. Then repeat the test. If 'colour' is still underlined right click on it and then click on 'Languages' and 'English (United Kingdom)'.
The other extensions I've got active include AdBlock Plus and Easy Youtube Video Downloader Express but you might have had different extensions (or none at all).
To install (or reinstall) any add-on, go to Tools > Add-ons > Extensions and pout the name of the extension you require (or a suitable search term, such as 'download Youtube') into the box at the top right. Then click on 'Install' next to the extension you want.
If Firefox is still running slowly after resetting, it suggest that something is hogging your computer's resources. In case it's malware, download, install and run the free version of Malwarebytes Anti-Malware (taking care, during the installation process, to remove the tick alongside the option to take a free trial of the 'Premium' version):
https://www.malwarebytes.org/antimalware/
(If malware is found, use the 'quarantine' option)
If that doesn't help, please refer to my post here:
http://www.theanswerbank.co.uk/Technology/Computers/Question1364249.html