Ttgjyuk

A recent update of Thunderbird in Ubuntu (version 60.2.1) broke the Lightning calendar installed as add-on. See here. Mozilla provides Lightning as bundled in the last versions, but Ubuntu (and Debian?) package do not, so now there is a mismatch between the Thunderbird version in the repositories and the add-on provided by Mozilla (5.4, only useful for Thunderbird 52).

One suggested option is to search and install beta releases provided by Mozilla, but it looks as a workaround (future automatic updates are compromised). Another is to delete the non-compatible Ligthning add-on and install a separate package (xul-ext-lightning) via apt, but it is not localised (only available in English since at least 2010).

So, which is the proper "future-proof" way to install Lightning in Mozilla Thunderbird from now on (as intended by Ubuntu maintainers)?

1. Remove the internal add-on (.xpi) and install (xul-ext-lightning) [this package will be updated accordingly, together with Thunderbird and, eventually, localised]


2. Keep the Mozilla add-on until the mismatch with Thunderbird apt package is fixed


3. Remove the add-on and wait until a future Thunderbird apt package provides it integrated (as Mozilla bundles it for other platforms)

Mozilla provides Lightning as bundled in the last versions, but Ubuntu (and Debian?) package do not [...]

yes, both do not, but they provide lightning via a seperate package (xul-ext-lightning resp. lightning) in their repositories.

debian also provides lang-packs (lightning-l10n-...) for lightning and ubuntu does not. --> that's IMHO the main issue.

[...] so now there is a mismatch between the Thunderbird version in the repositories and the add-on provided by Mozilla [...]

there isn't really a "mismatch"... for whatever reason mozilla simply does not provide lightning as add-on anymore.
EDIT: have a look at the last section of this answer.

(5.4, only useful for Thunderbird ≤52)

to avoid further confusion: lightning 5.4 is only intended to work with thunderbird 52.0!

there is no "one and only right solution" because every workaround has it's pros & cons and depends on your preferences and skills... nonetheless i try to prioritize them:

1. (if you are ok with lightning in english) the best is definitely to uninstall the lightning-addon and install lightning (xul-ext-lightning) via the repositories:
   https://askubuntu.com/a/1084095/354350
   

EDIT: at the latest now jump to the last section of this answer.

2. if you need a different language, probably extracting the e2fda1a4-762b-4020-b5ad-a41df1933103.xpi (=lightning-addon) from the official mozilla-releases (for every release) and installing into your profile via the addon-manager may be a reasonable workaround:
   https://askubuntu.com/a/1084220/354350
   


3. you can also use the archive provided by mozilla as a whole and update it on its own. - the packages (thunderbird* and xul-ext-lightning) from the repository should be uninstalled in this case.


4. _{if you are not afraid of potential bugs and do frequent backups of your data, installing the lightning-betas may also work for you:
   https://askubuntu.com/a/1084255/354350}


5. _{if you are a skilled tinkerer, you can perhaps use the debian-packages or parts of it (eventually with auto-update), but it is very likely you will end in dependency hell!}


6. if you are that skilled and seriously think about the last workaround, creating a PPA with lightnings lang-packs and providing them to all other users may be by far a better utilization of your time.


7. _{staying with an old thunderbird <60 for ages and using an already outdated lightning 5.4 is a really bad idea and should definitely be avoided!}

there may be 2 future-proof real solutions for all users, but i would not count on that we see even one of them soon:

• ubuntu should finally fix it's packages and include lang-packs into/for the lightning-package (xul-ext-lightning)! ...as debian does.


• 
  mozilla could probably release lightning as add-on again and stop forcing users to fiddle around with separately downloaded betas and so on...
  EDIT: since january 2019 (after nearly 2 years) mozilla resp. thunderbird (re)published lightning >=6.2 (for thunderbird >=60) as addon again!
  

Fix for lightning & exchange calendar

Thunderbird did an automatic update this morning to version 60.2.1 and my calendar (this is provided by work MS office365 (exchange) disappeared. I had been living on legacy Lightning and EWS plugins which were now incompatible. So after some effort I am now back up and running - this is what I did:

1) start Thunderbird goto Tools -> Add-ons and remove the incompatible plugins (greyed out) then quit Thunderbird

2) added xul-ext-lighning from repositories using Synaptic

3) restarted Thunderbird open Calendar pane, under the drop-down menu on right of tool bar I found an option to unsubscribe to the now defunct 'Exchange' calendar (I am not sure this is necessary but this is what I did) and I think you could do this directly from the Events and Tasks tab

4) goto Tools -> Add-ons and download and install the TbSync plugin, restart Thunderbird

5) Under the Edit tab select the new entry 'Synchronization Settings (TbSync)'; bottom left click button 'Account actions' -> '+ New account'; fill in details user name, password and exchange server (I had to use 'custom' option as TbSync could not automatically discover the work 365 server address); after a couple of minutes everything synced and calendar and address book were all correctly re-established.

One question I have is I am not sure how to set TbSync to automatically resynchronise or even if this is a problem.

Addendum - Due to change in TbSync, you now need to also download the Exchange Active Sync (EAS) plugin as well