I think this is an error from a web server. It might be the site you're visiting, or it might be a site that Firefox is trying to access in the background, such as an advertising server, or it might be a site that an add-on is accessing.

Could you try visiting some of those same sites in Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled (Flash and other plugins still work)

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

You can also try to clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"