It will be easier to get everyone onto Firefox than to some
"cutting-edge" Adobe applications. But no, everyone, please come to
the big red "o". 
And I don't see your point of having a forum to be ran as a desktop
application. Firstly, it becomes less accessible since users will
need to download the application before it is usable. Secondly,
coding the GUI for a desktop application is much more complicated
than cooking up a set of HTML and CSS code. Next, you have the OS
platform compatibility issue. I think it's best to just leave it as
a web application.
Besides, everything else is moving online as well. Now you already
have the online clone of Microsoft Word. Once we reach the age
where processors are measured in Terahertz and bandwidth by Gbps,
I'm sure you'll see young kids playing Doom XI without downloading
any client application for game installation.
To make your web application to be cross-browser, there are two
ways to go about it. Either use a well-developed framework that
handles all those tweeny-weeny browser issues, or do your own
browser checking and serve different code to each
browser.