Usually the file was copied into the wrong folder, or while Thunderbird was running, or it's too big for Thunderbird to index in one go. The fixes are below. If you only need to read or export the mail, you can also skip importing and open the file at readmbox.com.
Thunderbird doesn't have an "open file" button for MBOX. You add the file to its storage folder yourself, or use an add-on. The steps that work:
Alternatively, install the ImportExportTools NG add-on, which adds an option to import an MBOX file from a menu.
Thunderbird was open when you copied the file, or the file went into the profile's top folder instead of the Local directory folder. Close Thunderbird, move the file, and start it again.
Thunderbird may not have finished indexing, especially for files of several gigabytes. Give it time. If it stays empty, delete the matching .msf file next to it (that's Thunderbird's index, not your mail) with Thunderbird closed, so it rebuilds the index.
Very large Google Takeout files are hard for Thunderbird to index. Try splitting the file first: open it here, filter by year, and export a smaller .mbox for each year.
An Apple Mail export named something like Inbox.mbox is a folder. Copy the file called mbox from inside it, and rename it to something meaningful.
If you only need to read, search or export the mail, you don't have to import it into anything. Open it at readmbox.com: nothing is installed, nothing is copied, nothing is uploaded, and big files are read in small pieces. How it compares to Thunderbird.
How to Open & Read MBOX Files – 3 Easy Ways (Thunderbird section) on YouTube
Usually it was copied while Thunderbird was running, or into the wrong folder. Close Thunderbird, put the file in the Local Folders "Local directory" (shown in Local Folders → Settings), and restart it.
Not for the manual copy method. The ImportExportTools NG add-on makes it easier by adding an import option to the menu.
Large files are slow for Thunderbird to index. Split the file into smaller MBOX files by year first, or read it in the browser at readmbox.com without importing it.
Yes. Open it in desktop Chrome or Edge at readmbox.com. Nothing is installed and nothing is uploaded.
Ready? Your file stays on your computer.
Open your .mbox file