The plug-in seems to be requiring the permission to access the browsing history of the user. It's not immediately obvious why that is required for the plug-in. Why does the plug-in need access to the browsing history?
Topic on Talk:Wikimedia Apps/Reading list browser extension
Chrome displays this permission description as a result of the extension's requiring the "tabs" permission. You can see the details of that API here:
https://developer.chrome.com/extensions/tabs
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs
The tabs permission is required specifically to manage active/inactive state on Firefox (the extension is written so that the identical code can run in both browsers despite some API differences).
FYI, you can inspect the full list of permissions requested in the extension here, in the extension's manifest:
https://github.com/wikimedia/webextension-readinglists/blob/master/extension/manifest.json#L7-L18
Thanks. That was informative. The browsers could have given that permission string a little more thought. On reading that permission I thought the plugin had complete access to my browsing history (which seemed unnecessary for this plugin).