Most users update within a few weeks of new releases, but there will always be some that are using older versions. Certainly you will want to test with these three: Current regular release (36.0.1): https://www.mozilla.org/firefox/all/ Current beta version (37): https://www.mozilla.org/firefox/channel/#beta Current ESR release (31.5.0esr): https://www.mozilla.org/firefox/organizations/all/ Firefox 9 is very old by now. The previous ESR release was based on Firefox 24, so for companies (or individuals) that are very slow to update, you may want to test in the last point version of Firefox 24 as your "minimum" supported version. There's a link to old releases in the following article: [[Install an older version of Firefox]].