Selenium-Firefox: Firefox browser crashes when running my Selenium script

I was running some scripts I had developed in Selenium-Python today. They were working just fine. When I closed out a window & went to re-run a test, the Firefox browser that opened up would crash & my script would fail. This literally happened one second to the next. I wasn't sure what had changed to cause this.

I'm running Selenium 2.53 & Firefox 47 on my machine. Occasionally when I try to run my script again, I will get this run error in Pycharm:

File "C:Python34libsite-packagesseleniumwebdriverfirefoxfirefox_binary.py", line 98, in _wait_until_connectable raise WebDriverException("The browser appears to have exited " selenium.common.exceptions.WebDriverException: Message: The browser appears to have exited before we could connect. If you specified a log_file in the FirefoxBinary constructor, check it for details.

Normally I just get an error when I manually close the crashed FireFox browser that opens. Any ideas as to what occurred?