Both these scenarios could be related, but we'll start with a solution to the first.
If Firefox started hanging right after its installation, the problem could
be related to processors that use Hyper-Threading, such as Pentium 4 (with HT enabled!' The solution is a technical one: you'll need to update
the BIOS of your motherboard. It's best you let an engineer handle this. But before you call for assistance, go to your motherboard manufacturer's Web site to check whether new BIOS has been released for your model. It may not seem worthwhile going to all this effort just for a browser, but keep in mind that this problem might affect more than just Firefoxl

There's a workaround that you could try: Right-click the Firefox shortcut, select "Properties", and click on the "Compatibility" tab. Enable the option that says "Run this program in compatibility mode", and then select "Windows 98/Windows ME". The second problem you mentioned sounds as if it could be related to a buggy extension, a plugin, or another program that is interfering.
o If you just installed or upgraded an extension that is in beta phase or is buggy, uninstall it. Firefox user forums will be able to guide you through the exact specifics.
o If your browser is freezing after you click on a PDF, Flash, Java, Real media, QuickTime, or Windows Media Player file, it's certainly a problem with a plugin. Firefox should run fine if you remove the offending plugin.

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To remove the plugin, go to the directory where Firefox is installed (C:\Program Files\Mozilla Firefox\ pluginsl. and either delete the files or move them out of the plugins folder (see that Firefox is not open!. If you need the plugin, don't put the same plugin back; install a new one.

If your problem still remains unresolved, conflicting software (such as antispyware and virus scannersl could be the cause, Microsoft's AntiSpyware is a case in point. If you notice Firefox crashing or hanging when MS Anti-Spyware runs in the background, remember to avoid running the two programs simultaneously.
McAfee antivirus software too can stop Firefox from operating correctly. To fix the problem, open the "Virus scan console I Access protection I Prevent downloads from the World Wide Web", and add "firefox.exe",