This problem was reported during the thunderbird 68.* release cycle so the fix may affect older versions of Thunderbird negatively. The space (and possibly other non-ASCII characters, such as Umlauts / diacritics), would be url encoded (Space = %20) and could lead to failure of moving the mail or duplicated folders or non-working QuickFolders tabs. Drag to Create New Subfolders from tab could create unexpected results, if the entered folder name contained spaces.
If you are interested in using a version of QuickFolders that works with Thunderbird 78, buy a license to support the project. As in between step I plan to release QuickFolders as a 'web experiment' for Thunderbird 78+ but this will take a few weeks of work. I will also have to rewrite all mail Add-ons as web extensions, which is currently not yet possible as all necessary APIs to replace the deprecated tech has not been implemented. To keep having support sites and ongoing development, please buy a license (the migration will cost around $500, so there are some expense I hope to recover from the user community).
