Right now I'm using Xmarks to sync my firefox bookmarks among some computers I use (namely home and office).
Is there a way to sync these bookmarks between opera and firefox, so that I can use opera at home and firefox at the office, for example?
Delicious plugins or google bookmarks plugins on Opera and some other browser that has Xmarks so they sync from one to the other.
Another way would be manually exporting bookmarks from Opera and forgetting about Opera.
If you have Firefox 7, then export bookmarks from Opera bookmarks manager, in Firefox 7 import to bookmarks manager and set up Sync in Edit->Preferences and register and save registration code in gmail draft or some other save place.
You can also just keep your bookmarks in the cloud using something like Favsync so it's easier to access on any device.