Since update of Firefox v37 internet access is slow

Create a new Firefox profile to eliminate your preferences or Add Ons as being the slow culprit. If it's still slow, something's wrong with your Firefox installation.

Reinstall half of your Add Ons to see if one or more of them is slow. Use a binary search to determine which Add On it might be. Warning: More than one Add On might be a problem. Then you'll have to check each one individually.

If you use Firefox sync, you might try your profile on a new/different installation of Firefox (perhaps in a VirtualBox or a VMware Player guest).

My firefox takes almost a minute to load a page on every install (since Ubuntu 13.10 if I remember correctly). The only way to fix it is go to about:config (you write it where you write the pages url) and search for ipv6. Then change network.dns.disableIPv6 to true (maybe my router doesn't support IPv6 and causes firefox to search for it but it isn't needed yet so you won't have any problem and I think older versions had IPv6 disabled by default). After this, pages load within 1 sec.