What's your experience with forking Firefox?

Just wondering if anybody here has tried forking Firefox for the purpose of deep UI customization, and if you did how it went or is currently going. I haven’t been able to find much in the way of posts (on blogs or elsewhere) discussing the subject.

I have several browser UI design ideas I’d like to play with, and at first glance a Firefox fork seems like it might be a good option for this purpose. The primary challenge I imagine would be keeping the fork up to date with mainline for security patches.

Is this practical as a side project for a solo dev or will it be a struggle to keep up?