Hello there Tiger Rka07. We have read your message. We try to help you.

When mentioning a pop-up, something will indeed be asked to execute or as a mention a warning. To see if something went wrong when this pop up appears.

You can compress individual folders manually.

Right click on the folder in the folder pane and select 'compact' The folders that usually need this the most are 'Inbox', 'Drafts' and 'Junk'. see if this method helps and if the pop entry disappears.

Greetings Firefox volunteer.

I'm not sure that the last response actually addresses your issue. Hopefully the following will...

The prompt you received does NOT touch your Trash.

The process it is asking to proceed is normal maintenance, which for various reasons still offers the user an opportunity to cancel in case the timing is bad. But it is not intended that users should forever click cancel.

The recommended response is to mark the checkbox, and then click proceed.

Unfortunately the current language of the dialog is confusing. Version 91 comes out in several weeks, in which the language of the compact dialog has been greatly improved. When you install version 91, if compact causes problems, please create a new support topic.