A bit of a moving target because Firefox has just changed the menu labels (and hence some access keys) in bug 1690561, but apparently some of the changes are not yet in Thunderbird, PLUS they have reverted some changes in 1697682, PLUS there are user complaints in bug 1701324 about breaking their muscle memory by changing popular "Copy Link Location" access key from "A" to "L" (there's even an [addon](https://addons.mozilla.org/en-US/firefox/addon/copy-link-address-a/) to rectify that).
Bug 1710399 Comment 4 Edit History
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A bit of a moving target because Firefox has just changed the menu labels (and hence some access keys) in bug 1690561, but apparently some of the changes are not yet in Thunderbird, PLUS they have reverted some changes in bug 1697682, PLUS there are user complaints in bug 1701324 about breaking their muscle memory by changing popular "Copy Link Location" access key from "A" to "L" (there's even an [addon](https://addons.mozilla.org/en-US/firefox/addon/copy-link-address-a/) to rectify that).