もう少し詳しく...

Since Thunderbird extensions are rarely updated here is the simple the explanation how you can install them even if they are not updated for the current version of Thunderbird. Most of them work after such edit, but some don't (if some changes were made to Thunderbird that interfere with extension) . Do this at OWN risk tho.

1) Download extension.
2) Open (not extract/uncompress) extension in some archiver (program that you use to compress or uncompress files) like Bandizip, 7-Zip, ...
3) Find file "install.rdf" and use edit command on that file within archiver. It will open the file in text editor.
4) Find tag "em:maxVersion" and change number within that tag to set new maximum version of Thunderbird. You can use asterisk (*), for example "60.*" .
5) Save changes in text editor and exit.
6) File archiver should ask you if you want to update edited file within the extension archive (.xpi). Confirm that.
7) Install extension to Thunderbird as usual.

* Please use this only on abandoned extensions and at OWN risk. If an extension is still maintained, contact author for updates.