Hi, I'm running Firefox (both Quantum/Beta and main stable versions tested) on macOS Mojave (newest Beta). The behavior is the same - after File dialog is opened, I can do few actions (e.g. browse few folders, even successfully save) but if I leave it open too long, it stops responding causing the entire Firefox to freeze. Note that I can save or select a file as long as it is done quickly - "save as ...", then select folder from list of the recent ones and click save. But if you start browsing or creating new folder, or changing file name - it all freezes. The freezing happens regardless of website or whether it is “save” from Firefox menu or from a web app (eg Google Mail - saving attachment).