Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes).
See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears.
You can use "Disable all add-ons" on the Safe mode start window to disable all extensions.
You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

Hi and thanks for your reply. This does not help unforunetly.

What is the startup setting: Tools > Options > General > Startup: "When Firefox Starts" ?

Did you check the target line in the desktop shortcut ?

startup settings are normal, and nothing special about the shortcuts

Did you check the Home page setting?

Does the Home button work?

See also How to set the home page

Home page settings is normal, and everything works after I get that message.