Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of. Further information (Including how to remove it) can be found in the Troubleshoot Firefox issues caused by malware article.

In some cases, malware comes bundled with other applications, that seem to be harmless or even trustworthy. Especially if an application is provided as freeware, it is likely that it comes with hidden adware, browser hijackers or other types of malware. The best practice to prevent this malware from being installed on your computer in the first place is to always choose "custom installation", "I'm an expert", etc. during setup like here.

Dear Mario Thank you very much for the answer. I will check my computer with SuperAntispyware. I use to do that from time to time. I haven´t done that now for a while because I have not installed anything for a long time. Also I use to install by custom installation. I let you know later if I find something. Maybe it came with Firefox? That is the only software that i have installed recently that i know about :-)

Dear Mario You are a life saver! You were right! There where malware in the computer. I checked and cleaned it with SuperAntispyware and now everything is working just fine again. I have also saved the bookmark to the link you sent for the next time this happens. Maybe the superantispyware don´t finds it next time. So better I am prepared. And now that I recall, I remember it was the same issue last time this happends. My memory is good but very short some time.. Sorry for not remembering this until now. Anyway, thanks again! And I which i nice evening. Life is smailing again :-)