If your .csv file is opening in Excel it seems likely that you've browsed to it from 'Computer', in Windows Explorer, rather than within Thunderbird's import process.
Play the video from my first post and watch it from 1 min 3 seconds in. {There's a 'full screen' button at the bottom right of the frame, which will make it much easier to view). At about 1 min 57 sec, the guy locates the .csv file, highlights it and clicks 'Open'. The alternative way of doing that (which is what I gave you above) is simply to double-click on the .csv file. (So that's where you do the double-click, or simply use 'Open', not in Windows Explorer)
I have to admit that I only had a quick look at that video before adding it to my answer but, now that I've really studied it, I see that it's one of the most clearly explained Youtube videos that I've ever come across. (Give that guy 10 out of 10 and a gold star!). So if you watch it carefully you should be able to complete what you're seeking to achieve.