Evernote has expanded its read-later browser extension, Clearly, to Firefox. The extension first launched on Chrome in November. Clearly slides in a cleaned-up view of Web articles without ads or navigation, making content more pleasant to read. It automatically turns multi-page articles to single pages.

It’s also a content shifting tool. Clicking the Evernote elephant icon in the sidebar saves the cleaned up version to your Evernote account so it can be read on all devices. The article viewer also comes with three themes, and beyond that, all the fonts, colors and alignments can be customized.

Evernote thinks big, which is why it made our top 10 consumer Web products of the year. It wants to be a 100-year company, a cloud-based desk drawer for all our little files. It has recently shipped some interesting, unusual applications, including a food scrapbook called Evernote Food and a name remembering app called Hello.

Clearly brings Evernote into an increasingly crowded market dominated by dedicated read-later services like Instapaper and Read It Later. Like Clearly, Read It Later turns all articles into single-page views, but Instapaper intentionally doesn’t, in order to respect the revenue decisions of publishers.

Evernote Clearly could gain significant traction if users find they enjoy having all their cloud-synced stuff in one place, instead of having a separate app for reading. Content shifting is a new trend, and Evernote, whose basic service is free, is well positioned to introduce the behavior to new users.

Chrome and Firefox are the next biggest browsers after Internet Explorer, so Evernote Clearly is now available to a sizable chunk of the market. Chrome actually surpassed Firefox for the first time this month, only three years after launching, but Firefox renewed its search deal with Google yesterday, giving it a new lease on life.

You can install Clearly for Chrome or Firefox.

Do you use a clean-reading service? Which one do you use?