On Linux, I use Waterfox. On Android, I use Fennec. Both much lighter than Firefox, and Waterfox specifically doesn't need to be installed on Linux (my biggest concern in Linux - most is installed to the system), and the dev seems to be slowly separating Waterfox from the Firefox base, instead of keeping it as a stripped down Firefox. On the privacy side of things, not knowledgeable enough myself to audit them, but haven't seen any scandals on either yet.