Yes, everything needed is within your profile. If you have to restore the old profile, any new mail may be lost if you're using POP3 vs. IMAP.

I went through this exercise recently. It took several hours for the index to rebuild, and I had to restart Thunderbird several times when the indexing process seemed to hang. While all that was going on Thunderbird was very laggy and folders appeared to not refresh when I expected to. Eventually it all settled down and things again worked normally. This was on a slightly underpowered Windows machine, so it work faster/better for you.

I have lots of accounts with saved mail and the index had grown to almost a gigabyte. After the reindex the file dropped down to about 600 MB. I think the file had become corrupt, resulting in my search problems and the unusually large file.

It's my intent to initiate the re-indexing after I'm done for the night. I routinely shut off my internet connection when I leave my mac running at night. That's my plan unless there is some need for the re-indexing process to use server side resources via the internet .... it seems like that shouldn't be the case, but I'd rather know for sure without starting the re-index.

The size of my profile is significantly less than what yours is AFTER you re-indexed, so I'm thinking profile size isn't the reason global search had the problem I describe. Of course that's reasonable speculation .... which we all know can be unreliable.

Bottom line - do I need to be connected to the internet in order to re-index my profile?

I think the reindexing may need to download any message headers and/or bodies that aren't already present locally before it starts the process. Once that happens, it doesn't need a net connection any more. However, I don't think we have much control over what happens when. So, I would leave it connected to the Internet until the process is finished.

I mentioned that my index rebuild took a long time and kept hanging. I haven't heard anybody else complain about that, and some have mentioned that it too much less time than mine did, so maybe there's some peculiarity with my setup (although it happened with several computers in my house.)