Vote for https://support.mozilla.com/en-US/questions/832793

Try this:

1. Open URL about:config
2. in the filter box write: plugins.update
3. then set: plugins.update.notifyUser = false

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

Viewpoint Media Player is responsible for the plugin called "MetaStream 3 Plugin r4", and I believe this plugin is triggering Firefox to open the plugincheck page. It is not enough to disable the plugin, it must be uninstalled.

For me, after uninstalling Viewpoint Media Player, the "plugincheck always opening" problem then went away.

I did the same thing, and it worked! Thank you