Bugzilla

I think we are on the right track. What about:

Thunderbird needs to perform internal file maintenance to recover xxMB disc space.
Click on 'Proceed' to start or 'Cancel' to delay or to allow Thunderbird to do this automatically, select the checkbox below before you click on 'Proceed'.

It is short, uses maintenance says XXMB info and states all options. Personally, I'm not in favour of 'Cancel' because it does not Cancel, it delays and pops up again as soon as you delete an email.  A 'Delay' button would be more accurate.

The Options/Preferences > General > Disk Space says:
Checkbox "Compact all folders when it will save over xxxMB in total."
Selecting that checkbox implies it is automatic, yet it is not, it merely pops up a message to say that threshold has been reached so you need to compact.  Hence the pop up dialogue which is the topic of this bug.

Why is there no option / no checkbox to select  to do it automatically when you set up your options, yet the pop up message offers an option to do it auto from then onwards? 
Surely this area needs to sorted at the same time.
There needs to  be two settings/ checkboxes: 
The first sets the threshold as is the current state and offers the pop up dialogue, but at least it should mention it offers a prompt.  
The second is to set it up as automatic which seems to be only offered after getting a prompt and not in the place where you set up the options.
1. checkbox : Set threshold and prompt me to compact when it will save over xxMB in total.
2. checkbox : Select to compact automatically when set threshold is reached.

I think we are on the right track. What about this as it meets the requirements Wayne mentioned.
'Routine file maintenance' is not theatening and describes the overall process.
Instructions on what to do. 
One of the most frequent questions is how do I stop this pop up message because they keep clicking on 'Cancel' and it reappears as soon as they delete/move something.  The 'For more information' should not need to be an additional  click - I'm not in favour of additional clicks - stating the link to compacting article should suffice.

"Thunderbird needs to perform routine file maintenance to recover xxMB disc space."
"Click on 'Proceed' to start or 'Cancel' to delay.
" To allow Thunderbird to do this automatically from now onwards, select the checkbox below before you click on 'Proceed'.
checkbox:  “Do this automatically in the future.”
"For more information:"  link to Help Article on compacting

It is short, uses maintenance says XXMB info and states all options. Personally, I'm not in favour of 'Cancel' because it does not Cancel, it delays and pops up again as soon as you delete an email.  A 'Delay' button would be more accurate, but providing it is made clear that 'Cancel' means delay or 'not now', it's ok.