Is it the "When tabs don't fit" setting?

http://tmp.garyr.net/support/viewpage.php?t=3&p=display-tab-bar

Regarding how links open, are these links in a message displayed in a Firefox tab, or in an external email client?

For links in Firefox, usually you can divert them to a tab using the standard checkbox in the Options dialog:

orange Firefox button (or Tools menu) > Options > Tabs > "Open New Windows in a New Tab Instead"

However, when a link triggers a script, and the script applies special features to the new window, Firefox allows the links to be opened in a new window. This accommodates dialog boxes and other special cases. There is a hidden setting to override this and force those into a new tab as well if you think it's relevant.

For links in external programs, usually they follow the standard setting, but there is a hidden setting to deviate from that if you prefer.

In case I leave for a while, here are the settings I mentioned.

In a new tab, type or paste about:config and press Enter. Click the button promising to be careful.

In the filter box, type or paste link.o and pause while the list is filtered.

Double click these and set the value as desired:

(A) browser.link.open_newwindow

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

* First checkbox in Options > Tabs

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

0 = apply the setting under (A) to ALL new windows (even script windows)
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

I think there was some sort of bug, glitch or corruption. I deleted TMP and then reinstalled it & it reverted to the right settings. The tab rows reappeared. And you were right in saying where that setting is configured.

And I figured out that tab color is controlled in "Customize Styles."

Thanks for your suggestions.