This is the a quote of the general (FF4beta version) steps if you are too lazy to click another link:
1. Unpack the Firefox 4.0b9.tar.bz2 somewhere (e.g. /usr/local/firefox). With the "en_GB" release, I throw in a "dictionaries" sub-dir under there with en-GB.aff and en-GB.dic in there (and en-US.aff/.dic soft-linked to the en-GB ones) otherwise, sadly, Firefox 4 uses US spellings on what's supposed to be an en_GB release :-(
2. Download this 32-bit Fedora 9 libstdc++ RPM and unpack it with this command:
rpm2cpio libstdc++-4.3.0-8.i386.rpm | cpio -i --make-directories
3. Move the unpacked shared library into /usr/local/firefox thus:
mv usr/lib/libstdc++.so.6.0.10 /usr/local/firefox/libstdc++.so.6
Note: It's "usr/lib/libstdc++.so.6.0.10" above (i.e. the unpacked tree from the RPM, not the system /usr/lib tree) - do NOT put a leading slash there!
4. Run Firefox 4.0b9 with:
rkl references the beta version, but all steps still apply for official release.
I will say that FF4 is very nice! I am very impressed with what I have seen so far!