Hi,
unfortunately there are some comments in support forum about Thunderbird which is NOT belongs to portable versions. (i.e. *.xpi cannot install to portable).
Unfortunately the original and portable versions are mixed.
I suggest you to correct them - or improve portable versions to original version level. 
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properly done xpi will install with no problems to portable version.
However time to time some xpi did fail to install in the past, but I remember it was due to inability of such xpi to install properly to removable drive and had also then nothing to do with portable version as such.
This was mostly corrected with next update of the xpi.
Otherwise the here available version of TB is the original most recent version except it is packed in the paf. So it is the 'original version level'
From my experience so far, currently all xpi I use do install well and function well.
Do you have any specific xpi which does not work?
Otto Sykora
Basel, Switzerland
There is a comment this website among support forum "Using Thunderbird with Webmail". There is an information you need "Webmail extension". If you go there you can download different xpi files.
Below there are instructions:
How to Install
Right-Click the link above and choose "Save Link As..." to Download and save the file to your hard disk.
In Mozilla Thunderbird, open the extension manager (Tools Menu/Extensions).
Click the Install button, and locate/select the file you downloaded and click "OK".
In portable version there is not exist Extensions in tools menu.
That is why you cannot use xpi files in portable version.
Regards,
Brumm
The portable version and the local version work the same. Always have.
Also, the menu options mentioned no longer exist and have not in Thunderbird for some time. You need to click the menu icon (3 horizontal lines) and select Add-Ons to get to extensions now. If you use the old-school menu bar, Add-Ons is under Tools.
Sometimes, the impossible can become possible, if you're awesome!
have just tested and
can just confirm that the webmail extension does install and works under local and portable TB without any problems.
Otto Sykora
Basel, Switzerland