Sorry, there's not an easy way to copy or share the entire error log from Firefox for Android. To copy an individual log entry, you can press-and hold on the text to bring up a menu. This add-on has support for exporting various other debugging data from Firefox mobile. We've talked about adding support for the error logs in a future version. If you would like to help add that support, the source code is on GitHub: https://addons.mozilla.org/mobile/addon/nightly-tester-tools-for-mobil/