Have you tried asking Facebook support about that?

Yes. And I have received no response.

• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites that cause problems: Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems
• Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

cor-el, thank you so much for your answer. It worked perfectly.

For those of you who experience the same problem, it was the add-on called HTTPS-Everywhere that was causing the problem. I disabled that add-on and Facebook chat is now working.

You're welcome.