Using Windows 7 x64 build 7100 (Release Candidate) running 32-bit Firefox 3.0.10, this problem does NOT exist. Firefox is working fine on dual-monitors. This problem existed for me in Vista in 3.0.9 however. I'm not sure if something changed in Win7 but it's fine now. In case you need my useragent for version info:
Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.0.10) Gecko/2009042316 Firefox/3.0.10 (.NET CLR 3.5.30729)
Using Windows 7 x64 build 7100 (Release Candidate) running 32-bit Firefox 3.0.10, this problem does NOT exist. Firefox is working fine on dual-monitors. This problem existed for me in Vista in 3.0.9 however. I'm not sure if something changed in Win7 but it's fine now. In case you need my useragent for version info:
Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.0.10) Gecko/2009042316 Firefox/3.0.10 (.NET CLR 3.5.30729)