Crossposted from https://www.reddit.com/r/firefox/comments/1jd5nlk/convert_chrome_extension_to_firefox_addon/

There are a number of FF versions and forks that this will work with, but I used FF Beta (I've also confirmed it to work with FF Nightly & Iceraven. Conversely, it did not work for Mull and iirc, it cannot be done using FF Standard Release/Stable)

• Step 1 - have the CRX file for the Chrome extension handy. There are countless Chrome extensions and FF add-ons that will extract it for you.

• Step 2 - install the CRX Installer add-on (or get the XPI file by whatever means) →Extensions→Click CRX Installer→click "Browse"→ select the CRX file*, which should result in the creation of an XPI file.

• Step 3 - go to Settings→About Firefox Beta (or Nightly, Iceraven, etc)→Tap the logo at the top of the "About" section until you see a toast message saying "Debug Menu enabled" (I think 5 taps)

• Step 4 - navigate to "about:config" → search "xpinstall.signatures.required" →tap "Toggle" so that it displays "False" (make sure there's no whitespace; copy exactly what's between the quotation marks or the search won't return the corresponding flag)

• Step 5 - go back to the main Settings screen, and now under the Advanced section, after "Extensions" you should see "Install extension from file"→give that a tap tap taparoo (Happy Gilmore reference to lighten the mood 🤡)→select the previously created XPI file

• Step 6 = PROFIT!

💰💰💰

* I may have actually selected the .zip file converted from the extension CRX, but "conversion" in this case simply entails renaming the CRX file (i.e., change ".crx" to ".zip"). I doubt it makes a difference.

Edit - IDK why the link preview displays an add-on called WhatFont. The only hyperlink is for CRX Installer and I confirmed it redirects to its corresponding add-on store page ¯\_(ツ)_/¯

Edit 2 - I forgot to mention that this process will work to convert most extensions into properly functioning FF add-ons, but there will be the occasional outlier that doesn't function as well (or possibly at all) as a FF add-on due to differences in API calls. There are 1-2 that aren't available to both browsers and if the Chrome extension depends on one, functionality may suffer. Also, because FF is more permissive than Chrome in this regard, performing the conversion the other way around (from add-on→extension) is more likely to fail or result in a dysfunctional extension. Fortunately, due to there being a billion extensions in the CWS, most FF add-ons are already available to install.

Edit 3 - for a more automated approach, check out this handy tool created for the same purpose by u/hypeserver