Interesting as the path that works for you does not work for me.@OAT - That worked. Brilliant. I now see Thunderbird in the Send to list and clicking on it launches Thunderbird and attaches the file to a new message ready to send when ready.
There was just a slight omission in your Send to path but I am not complaining. I am happy.
You wrote %AppData%\Microsoft\Windows\SendTo
Should be %AppData%\Roaming\Microsoft\Windows\SendTo
As you can see I found the 'Send To' folder but there is no desktop.ini file so I cant check its contents. This does not concern me since my problem is solved.
Thank you all for your monumental efforts.
The path I indicated works for me. In order to see the .ini, you need to unhide system files.

My Computers
System One System Two
-
- OS
- Windows 11 Pro/All Channels
- Computer type
- Laptop
- Manufacturer/Model
- Acer Nitro ANV15-51
- CPU
- AMD Ryzen 7 7735HS 3200-4500 Mhz 8 cores x 2
- Motherboard
- Sportage_RBH
- Memory
- 32 GB DDR5
- Graphics Card(s)
- Radeon Graphic / NVIDIA GeForce RTX 4060 8 GB GDDR6
- Sound Card
- AMD/Realtek(R) Audio
- Monitor(s) Displays
- Integrated Monitor (15.3"vis)
- Screen Resolution
- FHD 1920X1080 16:9 144Hz
- Hard Drives
- Kingston OM8SEP4512Q 500 GB
WDS500G2B0A-00SM50 500.1 GB
- PSU
- 19V DC 6.32 A 120 W
- Cooling
- Dual Fans
- Mouse
- MS Bluetooth
- Internet Speed
- Fiber 1GB Cox -us & IGB Orange-fr
- Browser
- Edge Canary- Firefox Nightly-Chrome Dev-Chrome Dev
- Antivirus
- Windows Defender
- Other Info
- VMs of Windows 11 stable/Beta/Dev/Canary
VM of XeroLinux- Arch based & Debian 13 (Trixie)
-
- Operating System
- Windows 11 Insider Canary
- Computer type
- Laptop
- Manufacturer/Model
- ASUS X751BP
- CPU
- AMD Dual Core A6-9220
- Motherboard
- ASUS
- Memory
- 8 GB
- Graphics card(s)
- AMD Radeon R5 M420
- Sound Card
- Realtek
- Monitor(s) Displays
- 17.3
- Screen Resolution
- 1600X900 16:9
- Hard Drives
- 1TB 5400RPM