Firstly, did you put the firefox.app file in your Applications folder or did you run Firefox directly from the firefox.dmg file that you downloaded? If you run directly from the DMG file, a lot of users have issues with Firefox not properly saving their data. That can be addressed by installing Firefox properly. See How to download and install Firefox on Mac for more information.

If you installed Firefox properly, maybe the bookmarks that were imported are just in a folder. By default, Firefox usually imports your bookmarks from other browsers into a sub-folder of the Other Bookmarks folder. The folder will usually be called Imported from Safari or something similar.

To see if that's the case, press the Firefox menu button from the top-right corner of the screen and select Library. Within Library, select Bookmarks. Then click the Show All Bookmarks button at the bottom of the list. This opens the Library window. Click on Other Bookmarks from the left-hand side and see if your bookmarks in that folder or a sub-folder in there.

Hope this helps.

See Importing Safari data into Firefox

Related bug: Bug 1493103 Opened 2018-09-21 04:37 EDT Updated 2020-03-05 00:28 EST [Mac 10.14] Data Migration from Safari is not working due to lack of "Full Disk Access" permission

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AliceWyman, Thank you. I followed the procedure on giving full access from system preference, and I was able to import bookmarks. Thank you again!