As stated in the Canvas Beta Release Notes (2015-11-09):
When an instructor uses Crocodoc annotations for a supported assignment submission type, the Firefox browser frequently removes the last annotated comment when updating the score field in SpeedGrader. This behavior is a Crocodoc-specific limitation within Firefox that cannot be resolved by Crocodoc at this time. To warn about the effects of Firefox and Crocodoc annotations, Canvas displays a warning banner that says “Warning: Crocodoc has limitations when used in Firefox. Comments will not always be saved.” To avoid losing any annotations, instructors may choose to use another browser when providing Crocodoc annotated feedback, or try clicking the screen before navigating to another student.
This specific issue might not warrant switching browsers, but instructors making comments and annotations on documents displayed on the left-hand side of the screen in Speedgrader need to click outside of the last comment box they have authored to make sure it is saved properly before switching to the next student.
If you have any questions, feedback, or concerns, contact IT Academic Technology Services at the Faculty Commons (116 Pearson Hall, 302-831-0640).