Create the copy while you can see the list
In My List, select Create backup and copy all the text in the output box. Keep it in a note you control. Copying only the film names will not preserve the format used by the merge tool.
Merge rather than replace
Open My List in the receiving browser, paste into Restore backup and choose Merge backup. Recognised titles are added alongside existing selections. Checkpoints already saved in that browser take priority.
Read the result
A malformed backup is rejected. Unknown title references or invalid episode values can be skipped, and a backup labelled for another brand is not accepted. Review the message, then inspect the resulting list.
What you are keeping
The text contains viewing interests and manual reminders, not passwords, subscriptions or movies. It cannot recover a closed website's account history. Treat the copy as private if you do not want other people to see your saved titles.