The websites might be experience technical problems.
If the problem is with the browser try restarting the program. Try clearing all history. Finally, disable all add ons/extensions to see if that helps.
https://support.mozilla.org/en-US/kb/disable-or-remove-add-ons
“How to disable extensions and themes
Disabling an extension or theme will turn it off without removing it:
Click the menu button New Fx Menu and choose Add-ons. The Add-ons Manager tab will open.
In the Add-ons Manager tab, select the Extensions or Appearance panel.
Select the add-on you wish to disable.
Click its Disable button.
Click Restart now if it pops up. Your tabs will be saved and restored after the restart.
To re-enable the add-on, find it in the list of Add-ons and click Enable, then restart Firefox if required.”
Maybe there's something wrong with the Firefox profile. Try creating a new, clean, profile.
https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles
“Click the menu button New Fx Menu and then click Exit Close 29
Press Windows Key + R (click the Windows Start button and select Run... on Windows XP).
In the Run dialog box, type in:
firefox.exe -p
Click OK.
To start the Create Profile Wizard, click Create Profile... in the Profile Manager.
Click Next and enter the name of the profile. Use a profile name that is descriptive, such as your personal name. This name is not exposed on the Internet.
You can also choose where to store the profile on your computer. To choose its storage location, click Choose Folder....
Warning: If you choose your own folder location for the profile, select a new or empty folder. If you choose a folder that isn't empty and you later remove the profile and choose the "Delete Files" option, everything inside that folder will be deleted.
To create the new profile, click Finish.”