By Mozilla's lights, Firefox 3.6 has been a runaway success. The publisher of the open-source browser says that more than 100 million users have downloaded Firefox 3.6 since its launch in the middle of January. However, not all Firefox users have upgraded from Firefox 3 or Firefox 3.5, and Mozilla wants to change that.

Users of older versions of Firefox will start seeing this pop-up, asking them to upgrade. (Credit: Mozilla)

As of Thursday, users of older Firefox versions will start seeing a pop-up encouraging them to upgrade. The window will come with three choices: Ask Later, No Thanks, or Get the New Version. The pop-up will appear after 60 seconds of keyboard inactivity, which Mozilla called a courtesy toward users and their workflows. Selecting "Ask Later" will defer the window for 24 hours. If a user has chosen No Thanks but decides later to upgrade Firefox, running the "Check for Updates" option from the Help menu will bring up the upgrade window.

One reason that many users cite for not upgrading Firefox is a legitimate concern about add-on forward compatibility. In the press release announcing the push, Mozilla stated that more than 90 percent of Firefox add-ons are compatible with Firefox 3.6. Users can also try to force older add-ons to be compatible by using the MR Tech Toolkit or Nightly Tester Tools add-ons, which add a "force compatibility" option to the add-on context menu, but these tricks also decrease the stability of the browser.

If you use an older version of Firefox, tell us why and which version in the comments below.