See these articles for some suggestions:

• Firefox has just updated tab shows each time you start Firefox

• How to set the home page - Firefox supports multiple home pages separated by '|' symbols

• http://kb.mozillazine.org/Preferences_not_saved

cor-el, thanks for the reply. I went through the troubleshooting steps you linked, but they did not help.

1. I carefully checked my homepage settings. They are not to blame. I have only one homepage URL configured: http://www.google.com
2. I verified that my preferences are saving correctly, by toggling several Firefox and Add-On options, exiting Firefox, starting Firefox again, and seeing that the preference changes had persisted across restarts, and that prefs.js reflected the changes as well.
3. I disabled ALL of my Add-Ons, but the problem persists, even after restarting Firefox several times.
4. I tried a new, pristine profile, and the problem did not occur there. So, this is being caused by something in my main profile. I'd rather not abandon my main profile to resolve this, if possible.

Does it help if you (temporarily) rename prefs.js ?

You can also try to delete all the files and folder in cache location (parent folder of the cache as shown on about:cache).

C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla\Firefox\Profiles\<profile>\

hi cor-el. I deleted my cache folder as you instructed, and renamed prefs.js. When I do this, the first launch of Firefox shows:

1. The default homepage: http://www.google.com/firefox?client=firefox-a&rls=org.mozilla:en-US:official
2. The firstrun page: http://www.mozilla.com/en-US/firefox/3.6.17/firstrun/

The plugincheck page is NOT shown on this first launch (good). However, subsequent re-launches of Firefox do resume showing the plugincheck page on every start of the browser! All my addons are still disabled, and my real prefs.js is still out of the way, renamed. Very strange!

Update: I now also disabled all my plugins, in addition to disabling all my addons (I forgot they were two different things!) I then went back through the test (clean prefs.js, clean cache, relaunch browser. No difference! Plugincheck is still appearing on relaunch!

I uninstalled Viewpoint Media Player (and the AOL IM client AIM, since apparently that's what forced Viewpoint Media Player on to my system.)

For me, the plugincheck problem then went away.

I don't know why I installed AIM -- I know better than to do that! In the past I've used the open source IM client "Pidgin" to talk to people on AOL IM and it worked fine. And it doesn't have any advertisements, toolbars, or shady "media players" like AIM does. I'll go back to using Pidgin now! Bye AIM!

What is the value of the pref plugins.update.notifyUser on the about:config page?

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.
You can use the Filter bar at to top of the about:config page to locate a pref more easily.

My plugins.update.notifyUser has value default, which means false. I never messed with this setting during any of this, and this is the first time I've checked it.

It seems there is a bug in Firefox. Shouldn't this config value being set to false, mean that the tab should not open?

Aside from the possible bug in Firefox, my issue was resolved by uninstalling Viewpoint Media Player.

Bug 514327 - Detect outdated plugins and offer upgrade path

Download free and use Malwarebytes scanner. Visit majorgeeks.com for a lot of kool info on this stuff and the download. It worked for me 100% and it was free :o)