I tried this on 45.2.0 and I saw the same problem. I was not able to add an extension from a downloaded XPI file.

On 08/15/2016 12:09 PM, geo.inbox.ignored wrote:


On 08/15/2016 12:28 PM, jd1008 wrote:

in TB (Thunderbird for the un-initiate :)
Tools -> Add-Ons no longer provides  "install add-on from file.

Could someone show how to install an-add-on from file?

===>

your header for your post shows;

  User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) \
   Gecko/20100101 Thunderbird/24.0

which means your are not using a current version, therefore you should
upgrade to a more recent version.

on add-ons page, in upper right portion of page, you should see 2
selection bars. right most is labeled [Search all add-ons].

to left of that, a bar with a 'gear' and down pointing arrow head.
clicking arrow head produces a drop down selection menu with
"Install Add-on From File".

if you do not see such, then you _really_should_ upgrade to later
version of TB = Thunderbird.