Social search service StumbleUpon.com on Tuesday launched an add-on for Internet Explorer to boost its base beyond the estimated 1 million who use Firefox to stumble on new sites.

The San Francisco-based StumbleUpon, which in March closed its first round of financing after attracting an investor group that included former financiers of Google, Real Networks, and AskJeeves, sends users to sites recommended by others with the same interests. Until Tuesday, only users of Mozilla's Firefox browser could easily access the site.

"Over one million members of the Mozilla Firefox community are already stumbling to discover great sites," said Garrett Camp, chief architect of StumbleUpon, in a statement. "With the release of the Internet Explorer extension, millions more can start discovering interesting sites they never knew existed."

Once the IE extension is installed, a toolbar offers buttons that lead users to new sites, and lets them rate visited sites using thumbs-up/thumbs-down buttons.

The extension can be downloaded from the StumbleUpon site.