It's probably better to put off the launch for a few days to avoid making a really bad impression to potential users.
What you can take from this is while being IE compatible only will give you about 58% of users, you can see that FF is growing steadily and will probably catch up someday in the not too distant future. It also depends on what kind of web app this is. If it's something for younger people, or people more technology inclined, the FF visits may be a lot higher than the graph suggests.
It's a necessary evil. No one enjoys it, but if you don't make the fixes you'll prevent a lot of people (including myself, a FF user) from ever even trying your website. If I try it once and it completely breaks, I'm never going to come back.
If there are one or two non-critical features that are creating all of the bugs, then perhaps remove them initially and add them once you've fixed the compatibility bugs.
And users will figure out very quickly that it only works in IE; better to be up-front about it.
Personally, I can't imagine starting up IE just to check out a new web app, unless it seems insanely cool... so if you're looking at not alienating FF users, it's a good idea to get it working in FF asap (Firebug makes JS coding sooo nice).
IE-only means you lose all the Mac people, all the Linux people, and probably turn off the majority of the more clued-in Windows people.
Early adopters -- your target audience I assume -- don't tend to use IE.