I checked your problem, and my sincere apologies, but I could not reproduce the same problem here.
I use Firefox as default browser too (best browser, no?) on my notebook and visiting your site using FF, plugin did not show me the warning as if I were browsing in IE6.

Well, a explanation of how it works:
The plugin has a double check to determine which is the User’s browser, here we go:

First, a regex checks if the variable $ _SERVER[ 'HTTP_USER_AGENT' ] contains the “msie” string, if it is this part of the plugin to the next phase
If the phase 1 back positive, it will print the code that calls the pop-up beginning with , which should only be triggered by IE6 (or earlier)

I asked for a screenshot of your site and does not occur to activate the plugin using FF: http://browsershots.org/screenshots/483fee5eb03119ede6efa1067c285829/
Can you send me what the contents of the variable $ _SERVER [ 'HTTP_USER_AGENT'] are displayed in your FF?

I appreciate your willingness to help and thank you for reporting this issue, I hope to fix it with your help