You should definitely compact folders to avoid folder corruption, but the frequency, depending on how much space will be freed, can be set in the Disk Space settings in Options.

Defragmenting is best left to the automatic maintenance performed by Windows.

I understand that compacting results in copying the folders to new locations.

Where did you get that from?

You don't have to compact folders unless space is a problem, i.e, you have a low capacity SSD or are running out of space fast. The performance gains of compacting folders may not be realised when using an SSD. Did you see https://support.mozilla.org/en-US/kb/compacting-folders yet?

Thanks Stans. I got it from your link (or similar) . See:- "If the message is still current, it is copied to a new temporary MBOX file, called Nstmp. " Due to my concern about the limited writes on an SSD, I had set the default for compaction to 100MB. At the momment I have plenty of free space.

If I were you, I would turn off automatic compaction then only do it periodically or on occassion. How big is your Inbox file, for example, and how many messages do you download and delete, say, per day or week? A folder could be massive, spanning several GBs in size, yet very few messages get deleted from it (as is the case with mail horders), then you don't really need to frequently compact it because the gains would be minimal and not justify the writes spent on the SSD. You would be rewriting a 30GB file, for example, just to free up 200MB of space! On the other hand, a folder could be a few GBs in size, say 3GB, but gets a lot of messages coming in and out of it, that several hundred MBs of space get moved in and out of it in a short period. That folder's file would benefit from frequent compaction to keep it lean as opposed to bloated with messages that aren't current. However, If I had such a folder, I would not have it synchronized at all, but instead work with its headers only. All in all, it depends on your specific workflow, i.e, how many (and how big) messages get processed in and out of folders.

Thanks stans for your reply. I am learning. My laptop with the SD drive thunderbird has a number of different inboxes, generally kept small by deleting messages. At the moment the mail folder is 1 GB. I also use a desktop where messages are stored in Local Files (currently 3.75GB) but this is not on an SD drive so I allow auto compaction. It seems now is about time to compact the laptop.