Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Is this with all websites/pages, some, a few?

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem, Start Firefox in Safe Mode {web link}
by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh).
Did this help?

While you are in safe mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

Well, I did all the fixes you suggested -- which I'd already done once before. Yet again, the problem came back after a couple of days.

Gmail takes several tries to load at the outset, sometimes doesn't load at all, and often loses connection while using and won't respond. There is no error message, except for the latter, when it says that Gmail has lost its connection and will reload in xx seconds (which it never does.) Calendar works fine, but the news app doesn't always completely load. It only happens with Firefox. Safe mode does work, but why bother when all other browsers have no problems at all.

Start your Computer in safe mode with network support. Then start Firefox.
Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode
Starting Any Computer In Safe Mode; Free Online Encyclopedia

Are you possibly using external cleanup software (CCleaner) to clear personal data in Firefox as this can lead to corrupted files in the profile folder ?

Good idea -- I got rid of CCleaner about a year ago due to other glitches caused by it.