Red bar at top of screen "System update required-a critical software update is needed for your browser. Click allow to update" what is this???
Every time I go to a web site I get this red bar all the way across my screen, it cuts off part of the site I am at. Then Firefox puts a yellow line preventing the site from asking me to install software on my computer. With "Allow" only at the far right, the red bar has a blue arrow below the "Allow" button.
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Malwarebyte's Anti-Malware got rid of it for me.
This is a virus. Scan your systems for malware, clean up all found malwares. Now do a clean install of Firefox. You might save bookmarks if you like by exporting them before un-install and importing them after clean-installation.
You could use Avast! FREE AntiVirus or MalwareBytes Anti-malware standard if you don't have an antivirus installed.
All of the above are good suggestions. You might also read this Support article, which allows you to identify and remove malware:
Troubleshoot Firefox issues caused by malware
Hope that helps.