Yesterday, Google put up a post on the Chromium Blog to celebrate a year of extensions being available for their Chrome web browser. The main part of the post touts some big numbers that the feature has accumulated in the past 12 months. Those include, over 8,500 extensions, 1,500 themes, a third of Chrome users now having at least one extension installed, and over 70 million extension and theme installs total. But actually, looking at the Extension Gallery, the numbers may be even bigger.

According to the pages in the Extensions Gallery, there are actually now over 10,000 extensions in the gallery. 10,078, to be exact. The “Most Popular“, “Most Recent“, and the “Top Rated” areas point to that number. Each area shows what would appear to be accurate counts for the total number of extensions currently in the Gallery.

It’s not clear why Google wouldn’t tout the 10,000 number instead of the “over 8,500″ while marking their one-year milestone. But unless their Gallery counts are off, the number is into five digits now, just a year after launch. (It is worth noting that the Chrome Web Store does count just over 8,500 extensions. Presumably, the Chrome Web Store will eventually be the official home of extensions, but the standalone area does still exist with its higher count.)

And the number is also significant because Chrome’s chief rival when it comes to extensions (aka add-ons) is, of course, Firefox. So how many add-ons are available on that browser? Mozilla isn’t quite as transparent with the counts (instead, not surprisingly, they focus on cumulative download numbers), but presumably if you add up the totals from all the categories, you’ll get the overall total. As it stands today, that number is 12,739.

The last time we checked the two counts back in March, Chrome stood at just over 3,000 extensions, while Firefox had 11,623. So both are still growing, but Chrome extensions are growing much, much faster. At the current rate, Chrome would surpass Firefox in that regard at some point pretty early next year.

Firefox, which has had extensions for years now, is obviously still destroying Chrome when it comes to total add-on download numbers. But if you look at charts found here, you’ll see that since October, Firefox add-on download numbers have been dropping fairly quickly on a weekly basis. Perhaps this is as some users transition over to Firefox 4, which is currently in beta testing.

When Chrome was first released, users praised its speed, but many said they couldn’t switch from Firefox because of the add-ons. Google fixed that last year, and the numbers show Chrome gaining users at a much more rapid pace than Firefox is now. In fact, Chrome just because the top browser among TechCrunch visitors — ending Firefox’s four year reign.

Now the two are about to battle over web apps. Google just launched their Chrome Web Store earlier this week, and Mozilla is gearing up to counter when the Open Web App Ecosystem. Of course, as they stand right now, Chrome web apps essentially seem to act like either extensions or worse, just links to web pages hosting apps.

Chrome extensions are also an important part of the just soft-launched Chrome OS.

Seeing as Google is such an important part of Mozilla’s revenues, the two downplay this rivalry. But it very much exists.