Whether it happens only in firefox or Generally in windows too??

If it happen all the places in Windows, then its Mouse issue

• http://www.computerhope.com/issues/ch000819.htm

What happens if you press the right mouse button?

Did you check the settings in the Control Panel > Mouse to see if you have swapped the two buttons?

Sorry, I should have been more specific: it only happens in Firefox and I remember an option - somewhere - exists to enable this feature.

But I have 94 extensions installed and I can't remember which one has this feature, or whether it is a normal Firefox feature. I have searched all the likely extensions without success, and of course have "Googled" for it.

I remember turning this feature on, but can't remember where I did it!

I'm now regretting turning it on as it interferes more with normal operation than it is helpful.

The right mouse button works as normal - and shows the same context menu as the left button does when it is held down for a second (which is the purpose of the feature).

Mouse settings in Control Panel? - normal.

Hooray!! Found it! Thank you Google.

I found an old - very old - thread on Mozillazine.org:

http://forums.mozillazine.org/viewtopic.php?f=38&t=340572

And it IS a normal Firefox feature, but is normally OFF.

I think I must have recently turned it on after reading a "Firefox Tips and Tricks" article somewhere.

Anyway, the preference is called "ui.click_hold_context_menus".

And I just remembered that I added it in my "user.js" preferences file, amongst a load of others:

/*

*   Disable the Delay When Installing New Extensions
*/

user_pref("security.dialog_enable_delay", 0); /*

*   To turn off the URL greying
*/

user_pref("browser.urlbar.formatting.enabled", false); /*

*   To unhide the "http" portion of a URL
*/

user_pref("browser.urlbar.trimURLs", false); /*

*   Enable spell checker in multi-line text boxes & single-line text boxes
*/

user_pref("layout.spellcheckDefault", 2); /*

*   Double underline style
*/

user_pref("ui.SpellCheckerUnderlineStyle", 4); /*

*   Cycle through open tabs with Ctrl+Tab
*/

user_pref("browser.ctrlTab.previews", false); /*

*   Disable Animations for Full Screen
*/

user_pref("browser.fullscreen.animateUp", 0); /*

*   Click and hold on a hyperlink to show the context menu
*/

user_pref("ui.click_hold_context_menus", true); /*

*   Instant Apply Preferences Without Closing Options Window
*/

user_pref("browser.preferences.instantApply", true); /*

*   Middle-click in any textfield, addressbar or searchbox to paste clipboard
*/

user_pref("middlemouse.paste", true);

So there's a lesson to be learned here: don't add too many tweaks to your Firefox preferences at the same time or you may forget what you did!

Thanks, all. I'll now mark this as "Solved"!

The ui.click_hold_context_menus is usually only used on Mac because there it is possible that you have a mouse with only one mouse button.

• http://kb.mozillazine.org/ui.click_hold_context_menus