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

You could be experiencing bug 1166066 that happens when you are running Firefox in Private Browsing mode and you open links in a new window.

• bug 1166066 - Firefox 38 regression: link in private window with target="_blank" opens window with no tabs/toolbars/menus

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

• http://kb.mozillazine.org/Prevent_websites_from_disabling_new_window_features

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

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

Just to expand on that last part, in case you have never used about:config before, here's the step-by-step:

(A) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(B) In the search box above the list, type or paste feat and pause while the list is filtered

(C) For every preference that starts with dom.disable_window_open_feature. if it is not "true", double-click it to switch it to true.

This will affect other kinds of popup windows, too. You might prefer this anyway, since sites can no longer strip these features out of pop-up windows. But some sites may size popups on the assumption that they get the whole window and without accounting for toolbars. On sites like that, you may need to enlarge the window.

If the above does not help, Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?