Yesterday, Adobe officially contributed ActionScript 3.0's virtual machine (AVM2) to the Mozilla Foundation for integration into Mozilla (and, by extension, Firefox and Thunderbird). Within Mozilla, AVM2 will be known as "Tamarin". This means that at some point in the future...
* Mozilla's core JavaScript language will run faster (see initial performance tests)
* Mozilla's core JavaScript language (i.e., the syntax for defining functions, variables, loops, conditions, classes, interfaces, and so on) will be identical to Flash Player's core ActionScript language. ActionScript 3.0 (currently available in Flash Player 9) and JavaScript 2.0 (unreleased) are already both based on the ECMAScript Edition 4 draft specification, but Adobe's contribution means that the core features of both languages will eventually be implemented with same code base.
When is all this happening? According to the roadmap, JavaScript 2 (which will include Tamarin) will be released sometime in 2008. In the meantime, you can get used to the core syntax by playing with ActionScript 3.0.
Key Tamarin information:
>> Tamarin Overview by Frank Hecker
>> Collected Tamarin Links by John Dowdell
>> Official Tamarin project page
>> Brendan Eich's Tamarin announcement (Brendan is the original creator of JavaScript)