-> click Firefox button and click Exit

-> Start Firefox in Safe Mode by Holding down SHIFT key while starting Firefox

As an alternative method, press Win + R key combination and enter one of the following in the Windows Run box:

       firefox -safe-mode
       "C:\Program Files\Mozilla Firefox\firefox.exe" -safe-mode 

-> Firefox Safe Mode window will open -> DON'T SELECT ANY OPTIONS, just click Continue in Safe Mode button

Check and tell if its working again.

if it works, do this:

-> Update ALL your Firefox Plug-ins https://www.mozilla.com/en-US/plugincheck/

-> go to View Menu -> Toolbars -> unselect All Unwanted/Incompatible Toolbars

-> go to Tools Menu -> Clear Recent History -> Time range to clear: select "EVERYTHING" -> click Details (small arrow) button -> place Checkmarks on Cookies, Cache -> click Clear Now

-> go to Tools Menu -> Options -> General -> in Startup section click Reset to Default button -> place Checkmark on Show the Downloads window when downloading a file -> select Radio Button option2 Always ask me where to save files

-> go to Tools Menu -> Options -> Content -> place Checkmarks on:

1) Block Pop-up windows 2) Load images automatically 3) Enable JavaScript

-> go to Tools Menu -> Options -> Security -> place Checkmarks on:

1) Warn me when sites try to install add-ons 2) Block reported attack sites 3) Block reported web forgeries 4) Remember Passwords for sites

-> go to Tools Menu -> Options -> Advanced -> Network -> Offline Storage (Cache): click Clear Now button

-> Click OK on Options window

-> click the Favicon on SearchBar -> click Manage Search Engines -> select all Unwanted Search Engines and click Remove -> click OK

-> go to Tools Menu -> Add-ons -> Extensions section -> REMOVE All Unwanted/Suspicious/Incompatible Extensions (Add-ons)

-> go to Tools Menu -> Add-ons -> Appearance section -> REMOVE All Unwanted/Suspicious/Incompatible Themes (Persona)

-> Restart Firefox

You can enable your Known & Trustworthy Add-ons and Themes later. Check and tell if its working.