I have donated to some open source stuff (GIMP, Wikipedia, Qalculate, Firefox, Signal, Magit), though irregularly. More regularly things like
Unicef
UNRWA
SaveTheChildren
Greenpeace
Médecins Sans Frontières / Doctors Without Borders
Amnesty
I don't think there is a way to donate to Firefox unfortunately. Donations to Mozilla explicitly do not go towards browser development.