Here is my thought as a Firefox contributor,
Firefox Or Any other firefox builds my mozilla for,
1. Good selection of millions of addons
2. Sync Syncs EVERYTHING that you have done on firefox.
3. Mods available, Don't believe me? Go here, Btw this is my article
Collection of Useful Stylish Scripts to Customize Mozilla Firefox UI - AskVG
4. Stylish add-on to style websites can also style mozilla!
5. Search bar moved from address bar
6. Totally Customizable in every way
7. Don't like something on a page? Inspect it and set it to hidden so it doesn't appear
Cons
1. Not as fast as chrome, but still more customizable.
2. Can get slow if you mess with About:config
3. Chrome style UI as of April 2014 (use classic theme restorer)
4. No voice search
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Chrome
1. Simple UI
2. Less RAM use
3. Doesn't need to be restarted when you install add-ons
4. Chrome://flags is dangerous. Don't go there unless you know what you're doing.
5. Faster than Firefox
6. Voice search
Cons
1. Can leak RAM
2. Can become slow after awhile
3. Stole Firefox's "Australis" theme before they released it.
4. Firefox has wayyyyy more addons