What is your computer system and Firefox?

You may have corrupt xulstore.json or Localstore.rdf file.

Close Firefox. Open your file browser to this hidden location:

• Mac: *~/Library/Application Support/Firefox/
• Linux: *~/.mozilla/firefox/
• Windows: %AppData%\Mozilla\Firefox\
• *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\

Locate the above files. Then rename or delete them. Restart Firefox.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

I deleted all those files from the C-drive. Freshly installed FF. The results are the same.

I wrote I System restored Win7, which is Windows 7.

And your Firefox? There have been issues with v49 & 50.

I don't know. I guess for the moment, I just have to do without FF. At some stage I will see if there is a new version. In the meantime, I will use Chrome, which I hate, but which at least works. Just don't feel like spending much more time on this. So bye bye FF, maybe we will meet again in the future.

BTW, This issue has been coming up for years. I tried all suggestions of previous cases to no avail. It is unacceptable that a major software suffers from such failure, particularly since it was self-inflicted after I had to restart after an auto-update.

Sorry I do not know what is causing your issue.

I know I have seen quite a few reports of Firefox failing to start, or crashing and in many of those cases security software is involved, particularly AVG & Kaspersky, and probably more likely to be occurring when it is not the latest security software product. You may have to actually temporarily uninstall the security software instead of just disabling it to check if this is the problem

I am not sure Mozilla Firefox are even certain yet exactly which security software causes problems. An unofficial temporary workaround would be to install the special Firefox 45 ESR that has critical security fixes back ported and so is relatively secure. The simplest method of trying that would be to install the third party Firefox Portable as an additional browser either on a Memory stick or on the hard drive.

• http://portableapps.com/apps/internet/firefox-portable-esr

You could temporarily uninstall your security software on at least one of your computers to test if that is the cause of the problem. Please post back to say what you find and what the security software it is with its full name and version.

I know Mozilla developers are working on the issue, and IF that is what causes your problem I would hope there could be a fix from Mozilla or the security companies within a few days.

I uninstalled Avast with a cleaner, thus deleting all related files, but it still didnt work. I am not going to walk around with a browser on a pendrive, nor am I willing to spend more time on this. There are plenty alternative browsers. I am now using Chrome, which I like less, but at least it works. I still hope some time in the future FF starts working again on my laptop, but if not, power to the browsers that always work. Just for your info, I have started browsing with Netscape, to going to another browser is like closing a book for me, but I don't have time for things that don't work properly

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Start your Computer in safe mode with networking. Then start Firefox. Try Safe websites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

When clicking the link, my Chrome opens because FF does not start. I can't open FF directly because it does not open when using shift.

Type about:support in the address bar and press enter.

At the top right is a box with two links. press the one labeled; Restart With Add-Ons Disabled.