The fact that the problem is only happening in Firefox suggests that a setting, extension, or temporary file is causing the display to break on that specific site.

Reset Your Page Zoom Go to the LinkedIn home page in Firefox. Hold down the Ctrl key and press 0 (the number zero). This will reset the page to 100% zoom, which often fixes layout issues.

Disable Enhanced Tracking Protection Go to the LinkedIn home page. Look for a shield icon on the left side of the address bar. Click the shield icon, then click the toggle switch at the top of the pop-up to turn off protection for LinkedIn. The shield icon will get a line through it, and the page will reload.

Clear Cookies and Site Data for LinkedIn While on LinkedIn, click the lock icon at the left side of the address bar, click "Clear cookies and site data...". A small window will pop up asking for confirmation. Click "Clear." You will be signed out of LinkedIn.

Many thanks for your kindness and clarity, George!

Unfortunately after doing all 3 of your suggestions I still have the screen cut half as soon as I leave the main page on Linkedin: both if I go to a job ad or if I am going to "My network" page.... any other suggestions? Thanks! :)

@mmalagardis

You can also check if it is caused by an extension. Try to disable some

or use Troubleshoot Mode

go to the LinkedIn > menu > Help > Troubleshoot Mode… > click Restart

After you start Firefox in Troubleshoot Mode, test its behavior and see if the problem goes away.

https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode