The mobile Firefox settings page doesn't support this feature yet. However, you can create your own filter list and add it to the extension.
You can do that by creating a plain text file with the filters in it and uploading it somewhere (e.g. GitHub). In the settings page you need to then click on "Add a filter list" and "Add a custom filter list" after that. There you can enter the URL of the raw text file to add the filter list.
For example, here's a random filter list I found on filterlists.com which does that:
- GitHub project: https://github.com/k2jp/abp-japanese-filters
- Filter list URL: https://raw.githubusercontent.com/k2jp/ ... ty_sns.txt
Adblock Plus will automatically keep the filters in the filter list updated so it's also a good way to share your filters across all your devices and to share them with others. But that also means that your filters will be public so be careful when uploading your filters somewhere.