The Money Saving Expert website puts Now Broadband at the top of its current list but it's only available to customers who've not had a Now TV or Now Broadband contract before and only in some parts of the country:
https://www.moneysavingexpert.com/phones/cheap-broadband/
(Now TV/Broadband is, of course, part of Sky).
Try this tool to find the best deal available in your area:
https://broadband.moneysavingexpert.com/
Remember though that any service which comes through your phone line will be one which BT Wholesale has sold to your ISP. So there's not going to be a great deal of difference between different ISPs (except in their prices and/or customer support). If you're having problems using Firefox with your current provider they're not going to go away if you switch to a different one.
Try refreshing Firefox, as follows:
1. Note down the names of all extensions which you're currently using and wish to retain. (e.g. AdBlock Plus or a video downloader). To do so, click on the hamburger (three lines, top right) and then on Add-Ons. If 'Extensions' isn't already pre-selected, click on that too.
2. Click on the hamburger and then on Troubleshooting Information.
3. Click on Refresh Firefox (top right).
Refreshing Firefox takes the program back to its 'out of the box' state but retains key things like your bookmarks. However it removes any extensions you've installed so, lastly, . . .
4. Return to 'Extensions'. Then search for and reinstall whichever ones you want to keep.