The Free Software Definition (from the FSF) is:
A program is free software if the program's users have the four essential freedoms:
The freedom to run the program as you wish, for any purpose (freedom 0).
The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
The freedom to redistribute copies so you can help your neighbor (freedom 2).
The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
As you can see there isn't anything about plugins. Indeed, most programs won't even allow any plugin. If you want to change the software, you can, but you need to go back from the sources. In a sense, this will be the same with Firefox. Nothing will prevent you to modify the source and remove this restriction and run any plugin. So Firefox is still free.
Now I must say that @olliebulle's reasoning is appealing. You could consider that the Firefox released binary is, as a platform, and in some specific sense, nonfree. But the very fact that you can duplicate the platform very easily because it is software whereas you cannot when it is hardware is why tivoization is only a problem in the latter case.