If the browser interface is RTL, everything would be mirrored compared to an LTR interface:
As for the second part of your question, I wouldn't phrase it in terms of "good" or "bad" UX. It goes like this:
Usability and cognition
Part of usability is efficiency, which by many definitions involves (amongst others) the mental effort users have to expend in order to perform a task. Big part of mental effort is interpretation effort, and wrong interpretation leads to mistakes (errors due to mis-interpretation, as opposed to slips).
Interpretation is based on previous knowledge and understanding. Often this is referred to as users' mental model. So a good interface lends itself to its users' mental model (to what they know, understand and expect).
In turn, what users know and understand is based on previous experience, which is partly determined by conventions. Conventions are partly the outcome of the patterns in popular software.
So popular software only 'determine' high-usability in an indirect way - by promoting conventions, they provide designers with patterns that one can assume are known to users.
Consider, for instance, the arrangement of letters in the QWERTY keyboard:
The layout - dated back to a typewriter design from 1873 - was devised to reduce jam caused by different letter arms clashing.
This seems highly unusable: Why not just put the letters alphabetically, which all people are familiar with already. Yet, the layout became such a de-facto convention that it is now considered highly usable, whereas a keyboard sorted alphabetically isn't! If you have ever got tricked by some European keyboard that swap the A and the Z keys, you'd know how hard it would be to use a keyboard laid out as such:
Food for thought
There is no argument that physical interaction can be more or less natural. For instance, it would be terribly unnatural to have to pull the keys on a computer keyboard rather than pushing them.
However, you can argue that the only natural aspect of cognition is survival - everything else is either an outcome of the need to survive, or a historical, social or cultural convention.