There are two possibilities of this is happening.
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Updated browser
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Plugin or extension
1.) Updated browser updated
Sometimes, your browser gets updated automatically but you would not have updated your Selenium library. In that case your firefox binary will not support older version of Firefox driver.
To resolve that, you will have to uninstall browser and install previous version of Firefox browser or update your Selenium library
2.) Plugin or extension
If you happen to have new plugin or extension installed, there are possibilites that it will not let your browser function properly while invoked using Selenium WebDriver. In that case you will have to figure what plugin you installed lately and uninstall/disable the same.
Most of the time, your Antivirus or antimalware plugin should be suspected.
Hope that helps.
