Try simply closing Firefox and reopening it.
If that doesn't work, reset Firefox as follows:
Click the 'hamburger'. (Three lines, top right).
Click 'Add-ons'
Click 'Extensions'
Note down the names of any extensions you're using and wish to retain ('cos you're about to remove them!)
Click the hamburger again
Click the question mark (bottom line)
Click 'Troubleshooting information'
Click 'Refresh Firefox'
Return to 'Extensions', search for and install the extensions you want.