Firefox stops loading websites after a while

Question

Unfortunately I can't narrow down the problem very much. Firefox just stops loading websites after a while without any obvious reasons - while Chrome or IE work, so it's not a connection problem AND I have it on at least 2 PCs. This is my environment:
Windows 7 x64 Ultimate and Pro, all important updates installed. Both PCs have 8-16 GB RAM, SSD - can't be a resource issue.
Actual version of Kaspersky Internet Security is used, Firefox is not blocked.
Firefox version 37.0.1
Some FF AddOns in use: AdBlock Edge, BetterPrivacy, ColorZilla, Firebug, Flashblock, Ghostery, Web Developer
I'm certain I don't have a virus and the issue is present on a brand new installed system as well. I don't use a proxy, DNS prefetching is disabled and IPv6, too.
It seems to happen more often when the PC hasn't been touched for 20 minutes or so (PC is not falling asleep) and after sleep modes, however it also happens just in the middle of working. I usually have 10-20 pages open (resource friendly ones usually) at once.
This also happens for websites that are already loaded. A click on reload also leads to a can't load website error.
I can't exactly say when this issue started, probably in the last 6 month with some FF update. Before it worked fine. This is becoming a real issue for me because FF becoming very unreliable.
Does anyone else experience this behaviour or has a fix for it?
NB: you may want to check the compatibility of your "None of these solve my problem" button. It might be a Chrome (41.0.2272.118m) issue but if someone's FF is not working and he's trying to find help using another browser...it might be a dead end (nothing happens on click).
Example: https://support.mozilla.org/en-US/questions/new/deskto/other?search=can%27t+load+websites+after+while#summarize-question

Answer 1

Does Firefox work normally again after exiting it and starting it backup again?
Several users reported with Firefox 36 that Firefox would eventually not be able to load more pages. One believed that Firefox was no longer sending DNS requests, which is a prerequisite to being able to connect to the web server. I don't recall that we identified the source of the problem, but in many cases, it was "something" in the settings folder.
Along those lines, could you conduct this test:
'''Create a new Firefox profile'''
''This is in addition to and does not replace your existing settings.''
A new profile will have your system-installed plugins (e.g., Flash) and extensions (e.g., security suite toolbars), but no themes, other extensions, or other customizations. It also should come with completely fresh settings databases and a fresh cache folder.
Exit Firefox and start up in the Profile Manager using Start > search box (or Run) -- type or paste the following and press Enter:
firefox.exe -P
''Don't delete anything here.''
Any time you want to switch profiles, exit Firefox and return to this dialog.
Click the Create Profile button, assign a name like April7, and ignore the option to change the folder location. Then start Firefox in the new profile you created.
It might take a while to determine whether the new profile is works better than your old one. If you want to import your bookmarks from your regular profile during that time, here's how:
You can use the Library dialog (Ctrl+Shift+b or "Show All Bookmarks") to access the Restore feature. The trick is knowing where to find your bookmark backups in your regular profile. After clicking Choose File... you can paste this into the file name space and press Enter to drill down to your profile folders:
%APPDATA%\Mozilla\Firefox\Profiles
The semi-randomly-named folder that has your last backup is usually the one ending with "default". Click into that one, then into the '''bookmarkbackups''' folder. Here you can pick the most recently updated file to restore.
Important: restoring a bookmark backup wipes any bookmarks currently in your active Firefox profile and replaces it with the contents of the backup file.
I think it's also reasonable to install AdBlock Plus and Web Developer, as those, by themselves, are unlikely to be the source of this seemingly rare problem. But I wouldn't add all your extensions at once to the new profile.