My Mac has been having difficulty opening web browsers and other apps recently. I wanted to clear some login items. I found "Shift" When I try to check the box, the check quickly disappears. Could this be a virus?

First, download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. 

If that doesn't help go to this folder (HD/Users/Home/Library/LaunchAgents and see if there's anything in there with Shift in the file name. If so delete it.

Also look in the System/Users & Groups/Login Items preference pane so see if there's a Shift item in there. If so remove it.

f-secure is associated with anti virus software. It's not needed. There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community   

Download and run Find Any File to search for any files with the application's name  and the developer's name in the file name.  For f-secure software you'd do the following search(es): 

1 - Name contains f-secure

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

Would Shift be this application that purports to manage all your emails: https://tryshift.com? If so and if you are not using it, try to uninstall it.

Look for a Shift uninstaller in your Application folder. If there is none, from Finder > Applications, drag the Shift app to the Trash. If your Mac tells you the app is being used, force-quit it.

Click on the lock in the lower left, enter your password to unlock.

Then just select the item you want to remove from login items and click the minus sign - below to remove it.