This is a false dichotomy. Multi-process and sandboxing are sorely needed, so is a way to make the surface area available to extensions more stable/safe/secure.

This is a false dichotomy. Multi-process and sandboxing are sorely needed, so is a way to make the surface area available to extensions more stable/safe/secure. Waiting for a whole-cloth Servo-based browser to be totally competitive would be not learning from the not-so-distant past, where Netscape 5 was canceled in favor of the "correct" option of shipping the totally-rewritten Mozilla Suite. Netscape 4 withered away in the market, and it took additional years for the Mozilla browser to be ready and Firefox for it to find its footing. Making these changes to Firefox benefits users now, and deprecating XPCOM-based extensions in favor of Web Extensions helps pave the way to supporting other rendering engines, which will never support XPCOM (like Servo). In the meantime Rust is finding its way into Firefox, so code and lessons being learned in Servo (and elsewhere) can find their way to users sooner.