Short answer depends on the file type and how you are saving it (bare with me)

Per file type: Thunderbird tracks the last-used folder separately for each attachment file type. So if you save a .txt file, the next .txt will default to that folder. But if you then save a .jpg, it may jump to wherever you last saved a .jpg.

Save vs. Save All: When you use Save All Attachments, Thunderbird doesn’t look at the last folder per file type. It tends to fall back to the last-used folder from earlier in the session or, if none is “fresh,” to a folder it remembers from previous saves (sometimes higher in the path, which is why you see it jump up a level).

Session memory vs. history: Within a single Thunderbird session, it’s usually consistent. If you quit Thunderbird and come back, it sometimes resets or falls back to older remembered locations.

Because you’re mixing file types and using Save All, Thunderbird is bouncing between the “last folder used for this type” and “last folder used for bulk save.” That’s why sometimes you get exactly the folder you want, and other times it pops you into an older or higher-level folder.

What we can do to fix it: Set a default save location: In Thunderbird settings, under General → Files & Attachments, you can set a default save folder. Then, instead of Thunderbird remembering per-type, it will always start in that folder unless you tell it to do otherwise

ThundaMike said

Because you’re mixing file types and using Save All, Thunderbird is bouncing between the “last folder used for this type” and “last folder used for bulk save.” .... What we can do to fix it: Set a default save location: In Thunderbird settings, under General → Files & Attachments, you can set a default save folder. Then, instead of Thunderbird remembering per-type, it will always start in that folder unless you tell it to do otherwise

@ThundaMike: The intended functionality you note is buggy on Linux Mint 21.3. Thunderbird 140.3.0esr (and at least several earlier versions since last year) does not respect the preferred download location elected in Settings unless saving only one mail attachment at a time:

If an email has only one attachment, the preference is respected, but for emails with multiple attachments, setting a preference for a download folder has no effect when electing "Save All" attachments in an email with more than one attachment. In that case, TB opens the "home" folder and one must then navigate to the folder destination elected in settings.

Additionally, within the same mail session, saving multiple attachments from another email exhibits the same behavior, ignoring both the elected setting AND the last download location. ___ Edited to add that this behavior occurs whether the attachments to an email are of mixed filetypes (PDF, PNG, etc.) or are all of the same filetype.

Thanks for the explanation. I have to say TB seems a bit more consistent but there is still the difference between Save and Save All, as you explained. It is not entirely consistent with Save All though, it sometimes still chooses a folder I have not been saving to in weeks.

For my situation, where in most cases I would like to save to the previous location, keeping the current setting would be best. Otherwise I would have to change folders at every save. Now it selects the last one most of the times.

If there would be an option to set the location to Last Folder Used, independent of file type, that would be wonderful. I cannot think of a use case (for me, of course) where I would save in a location based on file type. So options for saving files would be Fixed Folder, Last Used (based on file type, will use Last Used for Save All), and Last Used (always last used, independent of file type, also works for Save All).