Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Did you check for hardware issues with your mouse (hanging left mouse button)?

On some systems, the mouse pad is the cause of the problem. Even if the pad is not in use, it sends a signal and . . . .

Some systems can be set up so that if a mouse is detected, the pad is disabled.

UnhappyBarb said

This has been going on for a few months now. At first, I noticed it in email and blamed Yahoo (my web mail provider). Then I noticed it in filling out forms on-line - like this one. When typing an email, I may move the pointer that looks like I (or the arrow associated with it) down so I can see what I'm typing, and find myself typing in the middle of my signature line; or move it up and suddenly be in the previous paragraph. In fact it just happened. The capital-I pointer was above where the cursor is (that indicates where I am typing). All of a sudden, I typing the next sentence in the middle of the word "blamed. I suspect it started with an up-date, but I can't pinpoint the date.

I was nervous about changing settings when I went into safe mode, but turning off hardware acceleration appears to have helped. Thanks. (I also looked at mouse and pad settings, but didn't see a way to change pad settings.) Thanks to you both for taking the time to answer. Barb