Do you use any security add-ons that could selectively block scripts based on the URL? NoScript is the leading example of such an add-on, but there might be others. If you use a proxy server in Firefox but not your other browsers, that also could filter your results.

Generally speaking, if you start up in Safe Mode, you can bypass most add-ons, so if that didn't work, you could try a follow-up step of testing with a new profile.

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, close all Firefox windows and start up to the Firefox Profile Manager as described in: Managing profiles.

Create a new blank profile and check those sites. Any change?

To switch back to your normal profile, close all Firefox windows, start up the Profile Manager, and choose that profile.

There is also security software (firewall) that can block JavaScript, so can can check there as well.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Firewalls
• https://support.mozilla.org/kb/Server+not+found

Thanks for your help. I created a new profile as you suggested, and indeed, in the new profile, the problem is gone - the sites I have problems with work properly.

So there is something in my default profile which is causing the problem.

Now how can I identify and correct the offending setting (or whatever it is)?

I have tried disabling all extensions and plug-ins, to no avail. Problem must be elsewhere.

You can check for proxy settings here:

orange Firefox button or classic Tools menu > Options > Advanced

then, on the Network mini-tab, check the Settings button for any custom settings

And... how about "third party" cookies? If you have blocked all third party cookies, try changing the setting to allow them for the current session. This article has some screen shots: https://support.mozilla.org/en-US/kb/Disabling%20third%20party%20cookies

Any luck?

Did you try to clear the cache folder on disk (about:cache)?

You can also try to delete some SQLite database files like permissions.sqlite and webappsstore.sqlite