The bad news: from Firefox 55 onwards, Selenium IDE will no longer work.
The reasons for this are complex, but boil down to two main causes:
Browsers are complicated pieces of software that are constantly evolving. Mozilla has been working hard to make Firefox faster and more stable, while still retaining the flexibility and ease of extension that we’ve come to know and love. As part of that process, Firefox is switching extensions from the original “XPI” format, to a newer, more widely adopted “Web Extension” mechanism.
The Selenium project lacks someone with the time and energy to move the IDE forwards to take advantage of the new technologies.
Ну, ок, я пока притормозил обновления Firefox, №54 работает норм.