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?

Two desktop units (WIN 10 Pro) and three laptops (also WIN 10 pro) have crashed, become erratic, and generally frustrated the patience out of me. Firefox works, other programs work, but when Nebula was installed in January 2025 everything went downhill fast. My folders would disappear; if seen would not allow editing of contents. Messages would 'vanish' without being touched. All of a sudden my "all mail" went from around 8,000 to 28,000. clicking on a folder to view it went from a double click to single touch opening and deletion. No longer able to highlight a message and move to a folder - now have to carefully mark as unread and then drop down through the 'move to' link to find a specific folder already listed on the left hand side. In short this is a pain. I have utilized and supported Mozilla and Thunderbird for many years. This is causing me to rethink this.