Suggestions:

• Customizable light/dark mode (and/or text/background colors)

• Option to remove the "Restore All Tabs" option entirely

• The amount of items to show in the list should be capped at whatever the maximum Sessions value is; at the time of writing this review, it's 25, but if there's a way to have it at the variable itself so that it follows it automatically without the author needing to update it manually, that would be perfect

• The amount of items to show in the list should maybe be 25 by default (or, rather, whatever the maximum sessions variable is, in any given Firefox version)

• Add a (removable) option to forget all listed tabs

• Allow holding the shift or control keys (or command key on Mac) to select multiple tabs from the list

• When right-clicking (a) tab(s) in the list, there should be an option to forget the tab(s)

• If possible, add functionality for recently-closed windows.
– Maybe listed tabs could be grouped by window.
– Upon right-clicking a tab, these options could be displayed in a context menu:
— "Open in Current Window"
— "Open in New Window"
— "Open in New Private Window"
— If the tab's window is currently closed (otherwise this option should be grayed-out): "Reopen Closed Window & Tab(s)"
– Maybe a listed recently-closed window could be right-clicked on to reopen all of its tabs, or left-clicked on to toggle collapsing it in the list.

While this addon actually works - it has some problem with its settings. Changing number of tabs to show in the list to anything more than 25 will cease addon of showing anything at all.
Also, keep in mind, that this setting doesn't override internal Firefox config `browser.sessionstore.max_tabs_undo` which is 10 by default. So, unless you modify the latter, the former won't show you more than 10.
Also, internal pages don't show icons in this addons' list, like Options (about:preferences), Addons (about:addons), other add-on's pages and etc.
And, lastly, FYI, Recently Closed Tabs is a native Firefox feature. You can find it under Library -> History -> Recently Closed Tabs. Library is a bookshelf-like icon on your FF toolbar. There's also Recently Closed Windows there.