Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Performing these steps makes firefox launch in a minimized window (filling half the screen) , then automatically goes to full screen. While there is no "flash," the above is different from a normal launch.

Launching in regular mode still creates the "flash."

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

Extensions did not fix the issue, however the hardware accelerator steps appear to have fixed it. Thank you for the remedy.

By changing these settings, will I encounter any additional problems with Firefox usage?

I don't think so. I always had my hardware off and no problem.

If you do have an issue later, and think it's connected, start a new question with a link back to this one.