Sure, you can donate money to develop Firefox, but it's not a core part of Mozilla's business model to support that. Instead you would need to track down an OSS developer that is either an active contributor, or who is willing to work on Firefox, and then sponsor their work through a number of different contributor pathways. There are Github sponsorships, direct funding, or any of the options here -
https://itsfoss.com/open-source-funding-platforms/It's not easy because supporting open source contributors is hard, and Mozilla is trying to keep their staff focused on corporate and foundation priorities.
Even if users could directly donate money to develop Firefox, the implication is that any such donations should only be used for Firefox development, when in practice, shipping a modern browser in a way that is competitive requires an enormous amount of non-"Firefox" related work (safe browsing, sync, telemetry, marketing ,release engineering work, advocacy, documentation, etc, etc, etc). Earmarking donations for engineering work is kind of silly in the context of an OSS project designed to compete with some of the largest juggernauts in the industry.