Just in case you are in full screen mode, try using the F11 key to switch to normal mode. (If that switches to full screen mode, use F11 to switch back.)

Normally Firefox behaves like any other Window. Although it isn't obvious whether this is caused by an add-on, or which one, you might want to try the standard diagnostic step of using Firefox's Safe Mode to see whether it is caused by a custom setting or add-on.

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the Safe Mode dialog, do not check any boxes, just click "Continue in Safe Mode."

Then see whether Firefox behaves more normally. If so, signs point to an add-on or custom setting issue. If not, hmmm, we would have to consider other factors.

Any luck?

Did you use one of these add-ons? - IDM CC* - Greasemonkey - Nightly Tester Tools - User Agent Switcher

Try to disable the add-ons one by one and check whether the problem is fixed... I solved this 'ALWAYS ON TOP' problem after disabled my IDM CC (v7.3.28) add-on. Firefox version: 15.0.1 (latest)

Right-click the Taskbar, choose Properties, choose Taskbar tab, make sure "Keep taskbar on top of other windows" in not unchecked.