I had a few (15) tabs open in an Orion window, and was opening more, but they didn't appear, so I kept clicking links, until I realised I had to scroll to reveal them in the tab bar.
Two things;
1) I think that Orion should scroll to reveal tabs as I open them
2) I think there needs to be a visual hint to show that there are more tabs scolled out of view. My idea would be to show a chevron/caret at each respective end.
Video showing invisible tabs until you scroll:
Firefox does it the way I would expect, showing a chevron at each end, and also scrolling to reveal newly opened tabs:
.