Hello Michal, try to Update Firefox to the latest version and check if the issue exist, if you have the same problem see: Unable to install add-ons or extensions.

thank you

See this article: Unable to Install add-ons or Reset Firefox preferences to troubleshoot and fix problems

Yes have now latest 24th firefox, have checked number of other addons, same problem.

I've noticed a couple of months ago or so, that the usual addon, to download the utube stopped working. I tried to install alternatives and until now not able to add any other addon after the no script add on. I even disabled temporarily my firewall to make sure if one is not blocking. No difference.

Now I attempted to install something like adblock or ghostery and things like that.

I only try to use the addon suggested by firefox. And I cannot install any not just one odd add on.

The message when ff refuses to install is something like that: It cannot modify the file needed...

So it attempts but it looks like some blockade is there or something got faulty. I've tried to allow all global scripts in no script add on, and also to disable my firewall. No difference.

Yes, I have the latest version of ff.

It is possible that there is a problem with the file(s) that store the extensions registry.

Delete the extensions.* files (e.g. extensions.sqlite, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.

• https://support.mozilla.org/kb/Profiles

New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.org/kb/Unable+to+install+add-ons

If you see disabled or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end side of the Search Bar to check if there is a compatibility update available.

You can also try to set the extensions.alwaysUnpack pref to true on the about:config page.

• http://kb.mozillazine.org/about:config

Thanks cor-el, perhaps here is a problem. In your suggestion I played with enabling files installation and I actually found the place but not the exact file "xpinstall.enabled", in the list were 3-4 other files similar to this and were highlighted bold and no string, no value in value field. The file "xpinstall.enabled" was missing as I searched manually and when I typed in a search box.

So I think that because this file is missing, I have problem with installing the addons.

In mean time I've reset ff and it made it worse. I've lost the only extension no script and am unable to install it again. My only solution would be to uninstall ff and install it again. Would it help?

thanks

No. The xpinstall.enabled pref is only present if you would create it (it is a hidden pref). When this pref is missing then it defaults to true and won't cause a problem. Even if set to false then Firefox will ask for confirmation when installing an extension or theme, so that wouldn't be a problem as well. This pref would only have effect when it is locked to false via a mozilla.cfg file (Administrators working for corporations might choose to do this).

Did you try to set extensions.alwaysUnpack to true?

Thanks for yet another suggestion.

I have located the extensions.alwaysunpack to true and it makes absolutely no difference.

Because of that, I do use Opera as my web browser though I prefer Fire fox.

I would really appreciate if this is solved successfully.

Thanks a lot

Create a new profile as a test to check if your current profile is causing the problems.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer some files from an existing profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

An old cliche: assume nothing Came handy in this case. Upon clearing my pc of only 33 malware items, magically the addons can be added without any problem. So the malware prevented me to install any of the security add on as I tried only add ons from this section.

I've added some security addons and it is quite disturbing that so many bugs and malicious ware are about everywhere including abc sites. It is very bad that so much time we are forced to waste on security while internet pirates are free to do whatever they want to.

Thanks for support