From a workflow point of view, integrating FIRE makes sense when there's a built-in material editor. In Softimage we use referenced materials because the UI toolkit is too limited to support the complex structure of the Maxwell material. In this case, since materials are edited with MXED, we can't update the FIRE window while you're tweaking a material. You would have to click edit, make a change and close MXED to see the change in FIRE, and that's hardly interactive.
We could probably build a material editor and embed it in Softimage using the platform UI API (Windows API, basically, since we don't support the plug-in on Linux anyway), but it would be quite a bit of work. However, we've taken a look at the event system in Softimage, and it's not good enough to support an interactive renderer. It's not even possible to be notified in a reliable way when an object gets deleted.
Given all this, we've decided we won't support FIRE in Softimage.
Edit: for the thumbnail issue, see if
this thread helps.