I have no Overflow Menu icon in my extension toolbar.
I had a bunch of addon icons in the toolbar, but they all disappeared for some reason. If I customize the toolbar, none of the icons are available to drag to the toolbar (maybe it thinks they’re already there?) If I remove then re-add an extension, then the icon appears in the toolbar, but there are some extensions I’ve made extensive changes to the preferences of or have accumulated a lot of data, none of which I want to lose by removing them. I’ve tried Restore Defaults, which works, but doesn’t resolve the addon icon problem.
I can’t guarantee it, but you could toggle these two preferences that [should] retain an extension’s local/random UUID between installations and the associated storage folder (the storage folder uses the local/random UUID in its name):
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
(2) In the search box in the page, type or paste onuninstall and pause while the list is filtered
(3) Double-click the extensions.webextensions.keepStorageOnUninstall preference to switch the value from false to true
(4) Double-click the extensions.webextensions.keepUuidOnUninstall preference to switch the value from false to true