Assuming you know what you're doing, maybe some script? At least on linux something like this seems to work:
#!/bin/bash
urls_file="${HOME}/path/to/url_list.txt";
sleep_time=1;
while read -r line;
do
firefox "${line}";
sleep ${sleep_time};
done < "${urls_file}";
edit: heh, tried to delete this as irrelevant, as I entirely missed the clipboard & requirement for a button IN firefox.. but it didn't really delete it seems. Oh well, leaving this in for laughs.
Anyhoo, if you need to speed up/throttle the link opening somehow, maybe add some incremental counter there and skip sleeping if counter < 10 or whatever.