The scroll bar works, but you can't scroll? What do you mean?

If you have a 3 button mouse, pressing the middle button in a blank area will turn the mouse into a scroll control.

This is Mac - not Windows.

Precisely - the mouse does not scroll the page = no on-page scrolling. The right side scroll bar is the only working scroll.

Symptom: when you attempt to scroll the page using the mouse "on the page", it doesn't scroll - but the right side scroll bars show.

On Mac the scroll bars can be hidden and only have them show when you scroll the page so you can see the current scroll position. The idea is that a touch pad is used that doesn't require the scroll bars to be visible and thus have more screen estate for the browsing area. You can make the scroll bars always visible: System Preferences > General > Show Scroll Bar > Choose "Always"

Are you running Firefox in (permanent) Private Browsing mode (never remember history)?

Possible workarounds:

(1) use Options/Preferences > General: "Open new windows in a new tab instead"
(2) Shift+click links to open in a new window with the standard features or use the right-click context menu.

You can modify dom.disable_window_open_feature prefs and change their value to true to prevent a website from disabling toolbars and scroll bars in a pop-up window

• dom.disable_window_open_feature.scrollbars
• http://kb.mozillazine.org/Prevent_websites_from_disabling_new_window_features

Hi cor-el,

No is answer to most of your questions. This has nothing to do with new windows or privacy - don't run private at all.

This is just about the sudden disappearance of "on page scrolling" using the mouse. Of course when I try to do that the right side scroll bars appear - and I have to use them!

ALSO NOTE: this is sporadic, about 90% of time. The other 10% the mouse scroll works! So not sure what this might be. Have restarted iMac several times and run FF on it's own and with other apps open.

POST NOTE: On page scrolling worked when I started reply to you, and then NOT working soon as I sent the reply.

Did you try to set some of the dom.disable_window_open_feature prefs to true?

Hi cor-el,

No, didn't try as it doesn't appear to be same problem. You described another problem entirely. Can you say why I should try it?

For anyone researching this problem, there's a solution from 3 years ago - which didn't work for me, but might for you. See https://support.mozilla.org/en-US/questions/870051 - "my 2-finger scroll isn't working".

I have just deleted Firefox 39 and re-installed from a fresh download. So far - the on-page scrolling is working. Will update this tomorrow to confirm, or not.

Confirming solution: fresh install of Firefox 39. On-page scrolling now works!

Thanks to everyone who gave advice.