Any chance you have the McAfee Site Advisor extension installed in Firefox?
If so, disable it until McAfee fixes it.

No, I do not have McAfee Site Advisor extension installed.

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")

I followed your instructions and it worked.

The add-on that was causing the problem was called 'Blacksheep'.

Here is what that add-on was supposed to do: 'Cloud-based security company "Zscaler" has launched a new Firefox web browser plug-in called "BlackSheep" designed to protect users against a recent browser hijacking tool that allows just about anyone to potentially hijack your web browser session."

I have re-enabled all my other add-ons and the bookmarks function is still working.

Thanks for the help.