when my computer has been on for some time, Firefox will not open without rebooting the computer. Is there a way to fix this?

I am using Windows Vista Home Premium on an HP computer. If I've been working for some time off-line (not on the Internet) and then try to open Firefox (double click icon or right click, then click OPEN), the system seemingly tries to open for a few seconds, then doesn't. I cannot open Firefox unless I reboot the computer. This has been happening with increasing frequency. Firefox is my default browser, but Internet Explorer is still installed in my computer, though I only use it now if Firefox won't open and I don't have time to reboot.