For me the about: addons tab will open after the last open tab. It is also true that I open the add-on manager always from the Firefox default home page.

You would have to check the settings in TMP to see how tabs are opened.

Firefox normally uses the browser.tabs.insertRelatedAfterCurrent pref (about:config) to determine where to open a tab, but TMP probably overrides this pref.

I try it on another computer with no TMP installed. It is the same. I forgot to mention: I'm not quite sure, but I think that behavior started with FF version 29.