If you want to help fund Firefox, you can for now just pay for a product https://www.mozilla.org/en-US/products/ and not use it (if you live in a country Mozilla accepts money from). Be vocal about it that you do this to support Firefox (e.g. reply in the discourse thread). I personally recommend leveraging MDN for this as it's right now the closest to Firefox, as in it's part of the Firefox organization within Mozilla. I would hope down the road we could just directly for Firefox, but we need to put money where our mouth is.