If you are using Firefox (or Mozilla), you should definitely upgrade to version 0.9.3 now because there are three critical bugs as well as the libpng bug in previous versions. Increasing popularity of Firefox means more vulnerabilities like this will surface.
While I think Firefox marketshare is moving up nicely, I don't think it will reach nor surpass 20% by end of this year. If you read my prediction carefully, I wrote that Firefox marketshare will be *near* 20%. Heh.