I have it on my system. No problem. Contact it's support site.

Your user agent shows that you run Firefox in compatibility mode for Windows 95 (NT 4.0).

Check that you do not run Firefox in compatibility mode. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

cor-el said

Your user agent shows that you run Firefox in compatibility mode for Windows 95 (NT 4.0). Check that you do not run Firefox in compatibility mode. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

still no joy.the following always pops up after clicking install now Video Download Helper could not be installed because Firefox cannot modify the needed file.thanks anyways

Does it say what file?

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

Delete the extensions.* files (e.g. extensions.json, 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

thank you FredMcD deleting the files you suggested has solved the problem. agan,thanks mate