Did you check the Home page setting?

See Tools > Options > General > Startup: Home page

See also How to set the home page

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

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

I saw the same problem last night and today.

Recently, the New Tab King add-0n was updated. Disabling the New Tab King add-on in Firefox fixed the problem.

If you have this add-on, try disabling it. Tools >> ADD-ONS

I notified them of this -- it appears to be a glitch.

Thank you tjlafave, I was able to sort it out as well, and I disabled the new tab king until they fix the glitch.