Donations to Mozilla don't fund Firefox development, they go to the non-profit foundation, which uses the money for things like open news, open science, policy work, etc.
If you have enough steady donation you can close Mozilla Corp which was created initially to handle business contracts with Google and a few other early business partners. Although Mozilla has many contributors, most of the bulk works are still completed by full time employees.
I believe Wikipedia runs entirely off as a Foundation, but I think they have attracted enough fund to support development and operation.
Perhaps it's easier for the general public to see the value in Wikipedia. The convenience, breadth, and availability of this resource as a repository of knowledge have never been rivaled. That's simply not the case with browsers. In recent memory, there have always been options, albeit meager at times, so the perceived cost of losing one browser may be significantly less.
I believe Wikipedia runs entirely off as a Foundation, but I think they have attracted enough fund to support development and operation.