You can try out Firefox 3.5 (not final yet) now and have a sneak preview of the new features.
Among the new features is the in-built support for video (there is a new video tag you can add to your (X)HTML pages)).
1. Therefore, grab a copy of Firefox 3.5 (pre).
2. When you run it, it is advised to run it as
./firefox -ProfileManager -no-remote
This asks you to select a different profile, so you can create a special profile just for testing Firefox 3.5. The -no-remote option helps you to have independent Firefox sessions from your normal Firefox you may be running.
3. Visit the Firefox 3.5 video demonstration page with subtitles.
4. Here is a version with translated subtitles for Greek.
Note that Firefox supports the OGV video container format. Therefore, you may need to convert your videos to OGV.