I think I learned about uBlock
from someone's sig here. It accomplishes the same goal as Adblock Plus/Edge, but uses significantly less memory. A freshly launched Firefox instance with Adblock Plus (only one empty tab open) used slightly over 230MB of RAM; with uBlock that figure is down to around 100MB.
I too first started using Firefox when it was called Phoenix, but I disagree that Mozilla has failed. In my experience Firefox is fast and responsive. Resource-minimalness (is that even a word?) isn't an issue for me, as I don't think Firefox requires unreasonable amounts of disk space or RAM. Especially after switching to uBlock. I suppose RAM usage starts to matter more if you like to keep hundreds of tabs open; it's a valid concern but not relevant for me.
(I couldn't figure out how to get the micro sign to work on Slashdot.)