Firefox writes to the disk often, but it should not be a bother.

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?

When is the last time you defragged your hard drive?

I tried this, but it made no difference. I also ran both an anti-virus program and an anti-malware program with no issues.

I checked last week, and my hard drive had very little fragmentation.