tools menu being referenced is Alt+T As a linux user it should be visible.

This is an old link On the toolbar http://chrisramsden.vfast.co.uk/3_How_to_install_Add-ons_in_Thunderbird.html Zenos has moved it but it works today.

As the images are taken from Linux Mint perhaps you can recognize the menu items better.

Okay, that's 2-3 bugs and an interesting article. I couldn't find the top-of-window menus because usually the alt key by itself reveals them but not in Thunderbird, which is one bug, and that there are two Tools menus, one at the top of the window and the other in the Thunderbird (hamburger) menu with not the same commands, is another bug. Top of window > Tools > Add-ons required a login into my Yahoo account but, even when I set Yahoo for logins by less-secure apps, Yahoo refused but may eventually have relented, which is a failure or a bug. I plan to follow up.

The alt key and F10 both work for me on at least two different linux boxes. Try alt+t as suggested.

I have never found any requirement to log in to an account before being able to access menus or the Tools|Add-ons page in particular. Your observations and comments are most puzzling.

Well if alt is not showing them, try F10 which is the correct thing for Linux. ALt is strictly speaking for Windows.

While you may consider having two menus with different entries a bug, the developers made a conscious decision to make it that way. It was not an accident.

Access to the add-on tab is not related to your mail and no log in to anything is required. Internet access however is required as the panel makes extensive use of remote pages and on server searches.

Alt-t worked before I last posted, but alt is normal as a way of exposing the whole row of menus in other programs, at least Firefox on the same Linux platform, and because it didn't expose them in Thunderbird I thought there were no menus up there, so I never thought to do alt-t until suggested in this thread. If you don't see the menus, you don't see the alt-combinations that will open them. F10 works but a user interface should apply users' common expectations, and alt may qualify.

Having two menus with the same name but somewhat different entries is confusing in terms of the user interface. If we use the program only occasionally, it's one more thing to remember which Tools menu to use. Most programs do not require that. I wonder what trade-off justified a harder-to-use interface. Space, maybe, but there seems to be enough space for the full manu in both places.

The login failure may have been a stray dialog in front of the add-ons tab but actually issued in connection with email access. I just reproduced it. That means another bug, because the dialog either should be more verbose so we'd know it may not be related to the current tab or should disappear from irrelevant contexts. The replication procedure was this: An email tab and the Add-ons Manager tab were open. The email tab was current. I clicked Get Messages, at the top. Briefly, nothing visibly happened. While nothing was visibly happening, I clicked on the Add-ons Manager tab, to make it current. Then, a dialog appeared, saying, "Login Failed"/"Login to server imap.mail.yahoo.com failed."/"Enter New Password"/"Cancel"/"Retry". While its wording is clear enough in this case, it appeared only while Add-ons Manager was current, so it presumably applied to that, implying that for some reason the Add-ons Manager wanted access to my email at Yahoo. I also wonder whether other email service providers have URLs that say "mail" and "imap" or some such; if not, the message could be even more confusing due to context. Perhaps the delay in the dialog appearing means TB was making multiple attempts to access online email, but it's still a UI bug.

I expect to try soon to get the add-on installed. Maybe it won't do it from the copy I already stored locally but only from an online source. I'll go back to the article you linked to above.