Importantly: The donations go to Mozilla Foundation, not to its for-profit subsidiary Mozilla Corp. MozCorp is who develop Firefox, the Foundation focuses more on political activism. If you want to help fund the people developing Firefox, one of the Corporation's paid products is the place to send your money.
I am not sure I want to know about your political opinions if you consider Mitchell as being "far-left" (not that I'm a great fan of hers). That is certainly not what the far left is, neither historically nor in the contemporary world.