Tab entries in the Vertical tabs are a bit too small

I don't fully agree with the tabs not needing more space horizontally.  When I have lots of tabs opened with similar titles (as might be the case when you're viewing multiple pages from the same site, and they all use some common long prefix), the narrow area does nothing to solve the problem that exists with horizontal tabs.

If the vertical line that's drawn between the tabs, and the "main" area for rendering the page, could be dragged like a splitter control, that would be ideal - that means you leave it to the user to decide what's appropriate or not.  On my 4K monitor, I can afford to allocate more space to it.  On my old laptop with much lower resolution, I might want to make it narrower.

If the browser picked a decent default width, but let the user resize it at will (and then remember it), I don't see how this wouldn't make everybody happy.