Otherwise you'll have a cash cow project that finances everything else. While that might be nice for one side, the other side would love to know that their funds are used to improve the already good product.
It's a holistic endeavor. Improving the product includes getting it in the hands of a lot of people, making decisions during conflicting ideas, writing better code, etc.
Restricted funding only hampers this on the perceived notion that the organization is stupid and will waste it all. There's no indication Mozilla will do this.
People who do not think like you should be taught to donate to restricted funds.
Why make people who would like to support development of the privacy first browser support decision of Mozilla to lease class A office space in a slew of expensive cities?
What I mean is that those limitations are made up. You just have to clearly state what you want to do with the money and that will fix those problems. Obviously restricted funds are nothing that is set up in 5 minutes, but I would expect that somebody really thinks about that.
And if you state that up to 10% of the restricted funds might also be used for any other projects of Mozilla, that might be okay.
Something like "your fund will be used for maintaining the infrastructure necessary to provide downloads of the latest binaries" would also be totally fine for me in terms of "buy a new server for this whenever you need it", be it a flooding or just the old server dying. But that obviously does not cover putting money into whatever shiny project is prioritized right now and might die in a year or less.
Otherwise you'll have a cash cow project that finances everything else. While that might be nice for one side, the other side would love to know that their funds are used to improve the already good product.