Many users are reporting a faster execution, but I guess that's not consistent. You can download a prior version and install over the top of the current version. After the install, there may be a compatibilities.ini file in the profile that may need to be deleted to avoid the message of having an incompatible profile. I presume you want ver 115, so here is the link, assuming English and 64-bit: - https://ftp.mozilla.org/pub/thunderbird/releases/115.14.0/win64/en-US/

Strangely - before I tried the fix suggested - the problem has resolved - however thanks for the helpful suggestion

@David, just remember that there will be changes to the profile data. Whatever was changed will represent data loss to the relevant user.

There is no official way of "going back" I suggest folk be referred to the feedback site to vent whatever feeling they have about that decision, because it as a consciously made decision in the Thunderbird V68 time frame.

Likewise the choice to opt out of upgrades was similarly removed. That situation will not resolve unless Thunderbird users make their feelings about lack of choice known.

Keith on 8-22 since wrote "since upgrading to nebula TB is very slow to open messages and also very slow to delet messages -unless this can be fixed I'd like to go back to the previous version which I think is 115 - anyone know how to do this proerly without ruining things".

So conditions changed?