Reading list data is 'reading-list.sqlite' file in the firefox profile that you used it in.

If you are not familiar with sqlite to do this on your own, but feel like doing so, download command line client from https://www.sqlite.org/download.html 
Copy both sqlite3 executable and reading-list.sqlite in one directory.
'sqlite3 reading-list.sqlite' will load it. command ".tables" should show "items".  http://stackoverflow.com/questions/6076984/how-do-i-save-the-result-of-a-query-as-a-csv-file#17938847  describes the way to export it, but replace 4th line's "select * from tbl1;" with "select * from items;".   Your reading list is now in "test.csv" file, you can import it in e.g. Excel.


If you want to just use browser (with no export function as far as I can see), you can use a firefox version that still has the reading list. If you use release/beta channel, copy 'reading-list.sqlite' file to its profile (note: mind your current reading list in that profile, if you have one in it too),
or you can downgrade your developer version temporarily.


To downgrade developer version:
Disable automatic updates first (options - advanced - update), otherwise old version will immediately update as soon as you close it.

I picked a version from https://download-installer.cdn.mozilla.net/pub/firefox/nightly/2015/08/2015-08-01-00-40-05-mozilla-aurora/   (that's 41.0a2, at least on of the builds with that id)
specifically https://download-installer.cdn.mozilla.net/pub/firefox/nightly/2015/08/2015-08-01-00-40-05-mozilla-aurora/firefox-41.0a2.en-US.win32.installer.exe  in my case
Reader list was present in that day's build.  When you use an older version, you can still use reading list feature and access your list items.

Don't forget to turn your updates back on when you are done.


(I tested 41.0a2, added items to reading list, updated to current dev release (reading list gone), downgrade to 41.0a2 - reading list was back WITH the items.
I also tested sqlite export and different profile method.)