Hi woll, would you prefer those new windows be diverted to a new tab in the current window instead? If so, you can try some preference change under the hood:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste neww and pause while the list is filtered

(3) Review/Edit the preference values as follows (double-click the current value to open an editing field, and then when you're done, press Enter or click the blue checkmark to save the change):

(A) browser.link.open_newwindow - for links in Firefox tabs

This is the one that has a checkbox on the Preferences page:

• 3 = divert new window to a new tab (default)
• 2 = allow link to open a new window
• 1 = force new window into same tab

(B) browser.link.open_newwindow.restriction - for links in Firefox tabs

By default, if a page sets width, height, or toolbars for a popup, Firefox will let it be a separate window. To force those into a tab as well, you can change this preference to 0:

• 0 = apply the setting under (A) to ALL new windows (even script windows with limited features) <= Change to this
• 2 = apply the setting under (A) to normal windows, but NOT to script windows with features (default)
• 1 = override the setting under (A) and always use new windows

(C) browser.link.open_newwindow.override.external - for links in other programs

• -1 = apply the setting under (A) to external links (default)
• 3 = open external links in a new tab in the last active window
• 2 = open external links in a new window
• 1 = open external links in the last active tab replacing the current page

To return to normal if things go haywire, there's an arrow-like reset button at the right end of each of those rows if it has a non-default value.

If you don't want Firefox to switch to that new tab right away and take you away from your current tab, here are a few more settings you may want to tweak:

(4) In the search box in the page, type or paste inback and pause while the list is filtered

(5) Review/Edit the preference values as follows (double-click the current value to switch between true and false):

(A) If the browser.tabs.loadInBackground preference is bolded and has a value of false, double-click it to restore the default value of true

(B) Double-click the browser.tabs.loadDivertedInBackground preference to switch the value from false to true -- this covers those cases of new windows diverted to a new tab instead

Success?

Let me mention three other things:

(1) I have no idea what your mouse buttons are doing.

If they simulate Firefox's right-click > Open Link in New window or Shift+click, then that will bypass the above settings.

(2) To avoid losing important windows from your session, exit Firefox using the menu. Either:

• "3-bar" menu button > Exit
• (menu bar) File > Exit

Then when you restore your session, all the windows that were open at the time you clicked Exit should come back as open windows.

(3) If you do accidentally close an important window, check the History menu. Either:

• "3-bar" menu button > Library > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

By default, Firefox remembers up to three closed windows in session history.