(Note, Firefox complained when I attempted to download it and claimed there was a virus, however, I converted the CRX file to ZIP using the CRX Extractor/Downloader extension from the Chrome web store, and when I compared the contents of the extracted folders, they matched almost exactly the contents of the extracted folders from the Firefox extension I had installed already. The only differences are the unique files that are required to submit to the Firefox add-on website vs. the Chrome Web Store, which are mostly text files and not executables.
https://chromewebstore.google.com/detail/crx-extractordownloader/ajkhmmldknmfjnmeedkbkkojgobmljda )
In Vivaldi, click 'Developer Mode' when viewing the extensions, then you can select the downloaded CRX file.
While I believe the file I downloaded today from crx4chrome is safe, that is not necessarily guaranteed in the future, so once ForecastFox is relisted in the Chrome Web Store, I would advise exporting your settings and reinstalling from there.