Since Firefox team has implemented the same browser extension API that Chrome has, we decided that we could also publish Web Scraper on Firefox. Without much effort we got Web Scraper running.
You can install the Firefox addon from here - https://addons.mozilla.org/en-US/firefox/addon/web-scraper/ . If there is something that doesn't work the same way it works in Chrome leave a comment here.
Also big thanks to Firefox team for making the web extension API.
Great news Martins! I've been waiting for it since Chrome Store Foxified stopped working properly since version 59 (again, due to Firefox team).
UPD: Storage Options page seem to be missing
UPD2: Altho it can be manually accessed via address bar:
moz-extension://d03e6106-3eaf-4149-9bdf-3b30da2a07a3/options.html#
(For those who also want to change storage options, the extension unique name can be different for you, and to know it, just hover your mouse over Sitemaps menu button)
Another method in case your extension ID is different, you have to type about:debugging into address bar, scroll down to WebScraper, click debug below it, then navigate to Inspector pane, where you can easily select it's internal URL inside <body> section.
UPD3: managed to import it into Opera, options page not available too (can be manually accessed)
Manifest file is just missing "options_page": "options.html"
Firefox doesn't support options page in manifest file.
It's not a big problem anyway 
Turns out it will skip entire page (data from it will be totally missing in CSV file) if it does not exist on a website instead of returning null data for particular URL in a sitemap.
Possible solution (as a workaround), is to add a selector that will pick 404 error on that page (haven't tested it yet tho, just an idea). Tested -- doesn't work.
i go to moz-extension://69aa905d-d20d-4ee5-9f95-dc5814572d66/options.html page manualy writing it. but still there is no couchdb option.
Here (in the last post of the topic) I copyed a working sitemap from chrome but then it totally not works on firefox, why?
I receive the same issue (firefox stopping to scrape immediately) with both windows 10 and linux mint. MAybe in some days I will try from Raspberry OS too, but I suppose to receive the same results without help form the forum..
url: https://vendiamos.com/uc/1-3-50k.html
sitemap:
{"_id":"ufficiocamerale-it-2vendiamos","startUrl":["https://vendiamos.com/uc/1-3-50k.html"],"selectors":[{"id":"name","parentSelectors":["links"],"type":"SelectorText","selector":"strong#field_denominazione","multiple":false,"regex":""},{"id":"pec","parentSelectors":["links"],"type":"SelectorText","selector":"strong#field_pec","multiple":false,"regex":""},{"id":"links","parentSelectors":["_root"],"type":"SelectorLink","selector":"a","multiple":true,"linkType":"linkFromHref"},{"id":"dipendenti","parentSelectors":["links"],"type":"SelectorText","selector":"strong#field_addetti","multiple":false,"regex":""},{"id":"fatturato","parentSelectors":["links"],"type":"SelectorText","selector":".OLDmb-3 li:nth-of-type(6) strong","multiple":false,"regex":""},{"id":"provincia-cciaa","parentSelectors":["links"],"type":"SelectorText","selector":"strong#field_ccia","multiple":false,"regex":""},{"id":"cod-ateco","parentSelectors":["links"],"type":"SelectorText","selector":"strong#field_ateco","multiple":false,"regex":""},{"id":"ateco","parentSelectors":["links"],"type":"SelectorText","selector":"strong#field_desc_ateco","multiple":false,"regex":""}]}