I don't understand well OpenStreetMap's philosophy. You make a free map, but it seems you use, to edit it, proprietary Flash technology Gnash cannot read. As far as Fiable.biz, my company, using 64 bits Linux, is concerned, we just cannot edit your maps, while we have no problem editing Wikimapia…
I support Vclaw's recommendation: Use one of the other popular OpenStreetMap editors. JOSM is compatible with the OpenJDK Java implementation available in many Linux distributions' repositories, and Merkaartor uses QT.
All major OSM editors are Open Source: Potlatch is available under a license that resembles a Public Domain dedication, JOSM and Merkaartor both use the GPL.
ActionScript 3 has been in existence since 2006. In these five years, the Gnash authors have not shown any inclination to support AS3. We cannot stick with an ancient language and runtime just because 0.1% of people use Gnash. The Gnash team are great guys, and P1 would not have been possible without all the work strk put into Ming, but I don't see why our technology choices should be bounded by what they choose to do.
Lightspark's AS3 support is coming on in leaps and bounds, and Lightspark is much more likely to support P2 than Gnash ever will. See http://sourceforge.net/apps/trac/lightspark .
You can download a 64-bit compatible Flash Player from Adobe at http://labs.adobe.com/downloads/flashplayer11.html .
Alternatively, Potlatch 1 is still available via the edit tab and still works with Gnash, or as Vclaw says, you can use JOSM or Merkaartor.