Yes. You can donate to the foundation but not only do they prevent earmarking, they actively don't use it for browser development.

I even mailed them back in June to confirm. They replied:

...

> When you donate to the Mozilla Foundation, your contribution goes directly toward advancing our mission to ensure the internet remains open and accessible for all. Our work focuses on issues like online privacy, open-source technologies, worthy AI and a digital world that puts people first. These funds directly support advocacy campaigns (i.e. asking irresponsible tech companies to protect your privacy), Mozilla’s fellowship program, MozFest gatherings, Common Voice, Responsible Computing Challenge, and so much more.

> However, it’s important to note that donations to Mozilla Foundation do not support the development of Firefox or any other Mozilla products.

> While we are a public-benefit 501(c)(3) organization under US law and the parent organization for the corporate entities that own Firefox, donations do not fund the Firefox browser and revenue is completely generated from within the product itself.

...