I've always been briefly annoyed at the new tab behavior when opening tabs on Firefox: namely that they open at the right most end of the tab bar. I think this decision probably has a lot of history but given that I'll have multiple tabs open at any time, I like to keep related tabs together, sort of a poor man's tree-style-tabs if you will.
Now even though there are a few extensions that fix this and I'm also thinking about writing my own :), you can simply change your about:config as well to enable this change.
browser.tabs.insertAfterCurrent in about:config to true. I'm using Firefox Beta 70.0b9 so this may not be available in the current release yet.