Chrome is a little faster but uses a lot more RAM. Firefox is a little slower but uses a lot less RAM.

If you browse with only one or a small number of pages open at a time, Chrome is best.

If you browse with 10 or more pages open at a time, Firefox is best. It uses so much less memory, in some cases it runs faster than Chrome because Chrome will consume all your RAM and start swapping long before Firefox does.

On a test I did today on my Ubuntu 15.10 system, with Chrome 46 running with about 12 pages open, overall system RAM usage was 10 GB (and the 8 GB system was swapping). When I opened the same 12 pages in Firefox 42 and closed all Chrome instances, overall RAM usage dropped to 6.5 GB and the system was no longer swapping, so it was much more responsive overall. That’s a savings of 3.5 GB of RAM with only 12 pages open!

The speed difference between Chrome & Firefox is not much on a modern fast system. If you have an older system the difference is more noticeable.

Of course you always need both because there are certain web sites that work only on one or the other. But which will be your default? After making Chrome my default for the past few weeks I’ve switched back to Firefox.