I propose the SysInspire Thunderbird to Outlook converter application if you wish to export Thunderbird to a PST file. This application may convert a single or more MBOX file to Outlook in a matter of seconds. It can convert Thunderbird files of any size into the required formats. Thunderbird files, including all folders and subfolders, can be converted to PST files. It includes a free demo version that converts the first 25 items from Thunderbird to PST files.