In terms of add-on availability, I would say it's an easy tie. Personally, I've yet to need one extension and not find it available across both browsers.
Add-on "discoverability" is also on equal footing between Chrome and Firefox, thanks to Firefox's add-on manager and the Chrome Web Store. Where the lines blur a bit, is that Chrome enjoys the added benefit of Web applications.
For example, Chrome has a Web shortcut "app" for Dropbox. In reality, this is simply a webpage shortcut. But Chrome considers it a web application. A second example would be AudioSauna. Once installed, both of these Web applications simply open up in a new Chrome tab. Firefox however, lacks this functionality. Some might be tempted to point out that this isn't a real loss, since the Chrome example is simply a link to a website running the Web application. But it's the convenience Chrome has provided in accessing Web applications that has many Chrome users won over.
After examing the Chrome vs. Firefox browser comparisons above, you might find yourself in a position where switching over from one browser to another might make sense. It's happened to me personally, a few times, so allow me to share some of the add-ons I've used to make the change as easy as possible.
One of the things I love about using Linux on the desktop is the amazing software choices we have available. This includes popular Web browsers found on the other operating systems as well. If you are trying to determine which browser is actually the "best" between Chrome and Firefox, then I recommend using the following basic formula.
If you have a reasonably new computer with ample resources, both Chrome and Firefox are good choices. However, if you're running with limited resources, I would suggest Firefox over Chrome.
And lastly, if you rely heavily on your Chrome extensions, but are considering switching back to Firefox, double-check to make sure your legacy extensions are going to be supported, as Firefox upgrades may not play nicely with their compatibility.