Mozilla’s support policy is an older version (in this case 1.5) of an application will (normally) be support for six months after the release of the new version. However there are exceptions, take for instance Firefox 1.5. Given this rule, support should have ended back on April 24th. However the final build (22.214.171.124) was not ready as anticipated so support will officially end when 126.96.36.199 is released on or around May 22nd.
On April 18th, Thunderbird 2 was released which means support for Thunderbird 1.5 will end on October 18th. Keep in mind TBird 188.8.131.52 will be released the same time Fx 184.108.40.206 comes out.
Now, some folks are wondering what exactly happens once support “ends” for Firefox 1.5? Simply put, Mozilla will no longer provide security and other updates for this version. You will still be able to get general support from the mozillaZine forums (note: mozillaZine is NOT part of the Mozilla Foundation). However, Mozilla (and as do I) recommend you upgrade Firefox 2. If some of new “features” of Firefox 2 are not to your liking such as say the new Close Tab Buttons, check out our Fx 2.0 Tweaks Guide.