I am trying to upgrade to Firefox 4.0 on a SunBlade 150 running Solaris 10 (sparc). Firefox 3.6.13 runs fine. But Firefox 4.0 (as well as beta versions 9, 10, and 11) hangs repeatedly. Sometimes it hangs the whole system; usually not. The ``Firefox hangs'' help page is not useful because Firefox 4.0 hangs in every circumstance listed there and every circumstance not listed there. It hangs when loading web pages (doesn't matter which) and when not loading web pages. It hangs with plugins enabled or disabled. Sometimes it yields ``illegal instruction'' or ``bad window parameter'' error messages, sometimes not. It will hang when loading any page requiring a password, or when loading a page not requiring a password, or if left alone for a few minutes. The only way to get it to exit gracefully is to quit immediately after starting it. Surely version 4.0 should perform better than this. What is missing, or what should be removed? Thank you for your assistance.