Multiprocess Firefox currently only uses one process for all tabs (plus a browser UI process). The tab process will be sandboxed, so users will get some of the security benefits of multiple tabs without the memory overhead of tab-per-process.
If you want to experiment with more than one tab process, you can tweak the about:config pref "dom.ipc.processCount".
Whichever one it is for chrome I have, it isn't ABP, it's a generic named Adblock. I don't use ABP due to the developer accepting money to whitelist google adverts.
AdBlock is significantly worst than ABP memory usage-wise (edit: on Chromium, I didn't benchmark elsewhere). I believe this is also the case CPU-wise from observing Task Manager during benchmarks.