Fastest check is to try a a new profile:

• Profile Manager - Create, remove or switch Firefox profiles

LazyDaisy said

I have Firefox Version 38.05 on Windows 7 and have refreshed, tried enabling a new HTTP cache.checked out the add-ons and still having the same problems The above statement about slow loading on my Firefox occurs on my desktop , but not on my laptop Windows 7. I Have tried all the recommended solutions and so far nothing works. Is there a possibility that some other programs could be in conflict with Firefox? This problem has been ongoing for several months and my patience is wearing thin. Any suggestions, fixes or advice? Thank you

Created a new profile and had the same problem, although I did not delete the original profile New info: Today I followed through and continued with the rest of the steps as advised in the previously mentioned advice. Sorry, but it did not solve my problem.

As a last resort I uninstalled my McAfee Anti Virus, and lo and behold all my slow woes with my browser have disappeared. I did install another anti virus (AVG) and all browsers appear to be working as they should!