Firefox for mobile (Fennec) is bumping its version numbering to more closely match desktop releases of Firefox. This means “Firefox 2.0b1 for Android & Maemo” is becoming “Firefox 4.0b1 for Android & Maemo” and will be released as “Firefox 4.0 for Android & Maemo”. We are aligning mobile Firefox and desktop Firefox since the web rendering engines used in both browsers are the same. Treating them as the same version seemed like the right thing to do.
Any add-ons hosted on addons.mozilla.org (AMO) that are marked as compatible for Fennec 2.0b1pre have automatically been bumped to support Fennec 4.0b1pre. If your add-on worked in Fennec 2.0b1pre, it will work fine in Fennec 4.0b1pre.