You can check the startup setting:

• Tools > Options > General > Startup: "When Firefox Starts":
  "Show my windows and tabs from last time"
  "Show my home page"
  "Show a blank page"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

It didn`t solve my problem. I have checked my startup settings and I have "Show my home page". When I started Firefox in Safe Mode it worked. Firefox did only open my home page not a new tab.

If it works in Safe Mode and in normal mode with all extensions (Firefox/Tools > Add-ons > Extensions) disabled then try to find which extension is causing it by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit)

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

I deactivated "Speedial 9.5.6" and it solve my problem with the new tab. Thanks for your help!