This is probably a dumb question, but what is GCC?
IIRC, GCC stands for the GNU Compiler Collection. Depending on how an app was compiled, it will need certain libraries. Some come with various apps, some do not. Many OS/2 ports/apps have dependencies on certain ones that only need to be installed once (and are shared by numerous apps/ports). Some such are the libc libraries (most notably libc063) and the gcc libraries. The readme file or web page for the latest OS/2 Firefox (which I have not yet played with) should tell you what you need.
Failing that, one can usually simply try to start the app from the commandline... go into it's executable's directory, type the exe name (in this case, "Firefox" or "Firefox.exe" - your preference), and it will generally tell you what DLL is missing.
And of course, so you dont have a dozen copies everywhere - and so you also dont have different versions (ie: an older one, and a replacement one with a bug fix, etc), drop the DLLs into a directory that's in your config.sys file's LIBPATH line. I tend to create a directory on the root of the boot drive named "DLLs" and drop all new DLLs in there that did not come with OS/2. Of course, I also added that directory to my path statement as well, so each app can find the dlls they need.
Best,
Robert