I have submitted a paper in a physics background journal about eight months ago. However, I have not received any status updated of the paper yet. I sent email several times to the Editor, but I have not received any reply. Five months ago, one of the journal staffs informed that they are waiting for the referees' comments. Does long 1st decision time indicate higher probability of acceptance ?
It is difficult to say so. Usually it means that the journal is having many papers to handle, referees are not responding in a timely manner or cannot find referees that are familiar with the subject of paper. Of course, I am putting aside the possibility that the journal may have a bad review process as it is usually uncommon for well-known and prestigious journals.
As a matter of fact, I had a paper that I had submitted with a prestigious journal. It took one year to have a first decision and the decision was rejection. In this case, the journal editor told me that the review process took a long time because they had difficulty with finding referees that were familiar with the particular subject of my paper.
This question is also somehow related to yours: Editor rejected my paper stating that reviewers had refused to review it long after submission...?
It depends. From my experience, any submitted papers will be under screening first. If an editor finds any obvious mistakes or is simply not interested in your paper, he will quickly reject the paper (often within a month) so that you can submit it to another journal.
If the paper can be under review for several months, it means that the editor is interested in your paper and serious about it. But this does not guarantee the acceptance of the paper. Sometimes reviews could take a longer time and rejection is still possible.