When a different program calls to Firefox, it opens a new window for it.

Fred, thanks, but TB used to just open a link in whatever FF window was available, without opening a new one. So where's the setting for "Open a link in an available Firefox window"? TB&FF behaved in that way for me for, what, a few decades.

Normally, yes, external links open in a new tab in the last active window. There are two possible reasons that isn't happening:

(1) Setting change (2) Separate profile launching

I'll give the steps to check on #1 below the line.

Regarding #2, if you notice differences in your theme, toolbar layout, or bookmarks between your regular window and the window launched by an external link, your default profile may have gotten out of sync with your preferred profile. Let us know if you need steps to fix that.

These are the settings for whether new windows get diverted to a new tab instead. For extension links, "(C)" is the most important, but please check the others as well.

(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 and determines whether targeted links are allowed to open in a new window or are diverted to a new tab instead; there's no effect on untargeted links:

• 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 open it in a "dialog-style" 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)
• 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

This preference governs those external links that open in Firefox (not directly relevant to your question):

• -1 = apply the setting under (A) to external links (default)
• 3 = open external links in a new tab in the last active window <== if the default doesn't work, try this one
• 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.

jscher2000, thanks. The possibility of two TB profiles occurred to me, so the instructions are welcome. More later.

OK, jscher2000, I followed the steps on checking/setting the neww defaults and changed (c) browser.link.open_newwindow.override.external setting from -1 to 3. Then I closed all FF and TB, found the odd FF window was the only one that reopened. From there I opened a new window. Then, clicking links in TB, I found that links would open in the *last-used* window of the two I had open. That's a good enough solution--better, I think, than messing around with TB profiles. So thanks!