If you get "Not a valid install package" - it is likely that the file you were trying to download has been truncated. I.E. The last bytes of the file are missing.
This can happen (for example) when downloading the file at work.
Presumably your organisation's security policy forbids this type of file from your network. Although the entire file is not filtered out, there is too much of it missing for .xpi to be usable.
It probably happens with a few other files too. See the solution...
Don't use the regular automated 'click and install method', as you have already found out - it doesn't work.
Instead use a dedicated downloader program to download the file to your desktop, or to your download folder - whichever you prefer.
Downloader programs can usually be configured to make repeated attempts to download the missing part of the file. Set this program to retry 8 or 10 times to complete the file download. If that does not work try setting an even higher number than this.
When the file is downloaded, double-click on its icon. - OR - Right-click on the file icon, then choose 'Open' from the pop-up context menu. Now it should auto-install into Firefox.
No guarantee - but it should work for many people.