This is the page I download from; the layout is what I expect.

https://www.thunderbird.net/en-GB/

This is what I get:

https://user-media-prod-cdn.itsre-sumo.mozilla.net/uploads/images/2020-12-08-09-54-54-1005bf.png

I think I may have made a mistake and upgraded to 84.0b3 when I should have stayed with 75.xxx.

Can I downgrade my 84.0b3 and still keep all my settings and data?

Any more advice please?

Don't use mozbackup, it was abandoned many years ago. Use https://support.mozilla.org/en-US/kb/installing-thunderbird-windows/discuss/8259

I suggest you do one thing at a time, transfer to the new PC first, and install the current beta version.

please see these pages on Options On Regaining TB Data ( 1 : downgrade , 2 : second-TB , 3 : transfer data , etc ), those might+should be helpful. make a backup of your TB-"profile" 1st, then try what you want+need to do.

Thank you for your replies. There is a lot I need to read before taking any more action; some I have taken may/will cause me problems.

I have taken a copy of the profile. However I then decided to uninstall my 84.0b3. This was a bad decision I am sure!!

Among other things I need to get hold of an .exe of 84.0b3 so that I can return my current pc to a working Thunderbird state; having uninstalled 84.0b3 nothing works. If I install 78.5.0 I get the error message that I need to create a new profile. The method to get round this, using CMD, I am not sure I fully understand yet. How to add the --allow-downgrade parameter to a cmd command. I have used cmd in a limited way in the past but I am unsure I will type the right command.

I think I understand this bit:-

cd Thunderbird installation directory ./thunderbird -ProfileManager

but do I just type:-

cd Thunderbird installation directory ./thunderbird -ProfileManager --allow-downgrade

However this is not to be tried until I understand and maybe even get running your instructions first.

I got lost reading those instructions so took note of:-

"IF INSTRUCTIONS ARE (STILL) NOT CLEAR TO YOU , THEN DO NOT FOLLOW THESE STEPS."

and took the cowards way. CMD option

CD \Program Files (x86)\Mozilla Thunderbird ENTER

Thunderbird -ProfileManger --allow-downgrade ENTER

Now have a working 75.5.0 Thunderbird on my old machine.

Using the copied profile I now have Thunderbird 75.5.0 working on the new laptop. SUCCESS!

Hope this works and may help others.

BTW Backup problems. First backup onto 4TR HDD on machine. Next tried to copy the 24GB profile onto an empty 32GB micro SD card; at 19GB copied message "not enough space on card to copy Inbox"; was able to copy to an empty 200GB micro SD card and another 2TR HDD.

in Win10, path of TB exe, need to be used with thunderbird.exe CMD, it should be like this:

32bit TB: "C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe" -P --allow-downgrade 64bit TB: "C:\Program Files\Mozilla Thunderbird\thunderbird.exe" -P --allow-downgrade

then thunderbird.exe goes into "%APPDATA%\Thunderbird\Profiles\" and uses one of the TB-profile folder from there : xxxxxxxx.<profile-name>

which-ever backup copy of ALL profiles "%APPDATA%\Thunderbird\Profiles\" succeeded in full-copy , keep that one only for now , multiple copy of backup is not needed. when all is working fine, all data is intact , then delete backup copy.

when TB deletes an email, actually it still keeps it with other emails but marked as hidden,etc. in this way TB-profile become huge. as you have backup copy , you can try "Compact Folder" option ( with your own decision to take the risk, that data loss may occur ) , if you have not used it that much . but know this, "Compact Folder" will completely ERASE hidden emails, etc . only your backup will have a backup copy , if for some reason you want to recover something that was erased , recover will not be very easy. compact-folder result, cannot be undone. TB will run little bit faster after this.

Thank you

Will run a backup on the new laptop and get rid of all old stuff.