Instructions in readme section "Development" do not work. Cannot build the plugin

[schra]

• Multi-Account Containers Version: master (f0afc0d)
• Operating System + Version: Arch Linux
• Firefox Version: 84.0
• Other installed Add-ons + Version + Enabled/Disabled-Status: I disabled all plugins but Multi-Account Containers

Actual behavior

$ npm install
npm WARN ajv-keywords@3.5.2 requires a peer of ajv@^6.9.1 but none is installed. You must install peer dependencies yourself.
npm WARN webextensions-api-fake@1.3.0 requires a peer of sinon@^8.1.1 but none is installed. You must install peer dependencies yourself.
npm WARN webextensions-jsdom@1.4.1 requires a peer of sinon@>=8.1.1 but none is installed. You must install peer dependencies yourself.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.2.13 (node_modules/chokidar/node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.13: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@2.1.3 (node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@2.1.3: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@2.0.1 (node_modules/web-ext/node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@2.0.1: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})

audited 1688 packages in 6.261s

92 packages are looking for funding
  run `npm fund` for details

found 14 vulnerabilities (9 low, 5 high)
  run `npm audit fix` to fix them, or `npm audit` for details

$ ./node_modules/web-ext/bin/web-ext run -s src/
(node:41062) Warning: Accessing non-existent property 'cat' of module exports inside circular dependency
(Use `node --trace-warnings ...` to show where the warning was created)
(node:41062) Warning: Accessing non-existent property 'cd' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'chmod' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'cp' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'dirs' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'pushd' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'popd' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'echo' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'tempdir' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'pwd' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'exec' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'ls' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'find' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'grep' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'head' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'ln' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'mkdir' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'rm' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'mv' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'sed' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'set' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'sort' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'tail' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'test' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'to' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'toEnd' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'touch' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'uniq' of module exports inside circular dependency
(node:41062) Warning: Accessing non-existent property 'which' of module exports inside circular dependency
Applying config file: ./package.json
Running web extension from /home/andre/Git/multi-account-containers/src
Use --verbose or open Tools > Web Developer > Browser Console to see logging

WebExtError: Temporary add-on installation is not supported in this version of Firefox (you need Firefox 49 or higher). For older Firefox versions, use --pre-install
    at FirefoxDesktopExtensionRunner._callee6$ (/home/andre/Git/multi-account-containers/node_modules/web-ext/dist/webpack:/src/extension-runners/firefox-desktop.js:273:19)
    at tryCatch (/home/andre/Git/multi-account-containers/node_modules/web-ext/node_modules/regenerator-runtime/runtime.js:62:40)
    at Generator.invoke [as _invoke] (/home/andre/Git/multi-account-containers/node_modules/web-ext/node_modules/regenerator-runtime/runtime.js:296:22)
    at Generator.prototype.<computed> [as throw] (/home/andre/Git/multi-account-containers/node_modules/web-ext/node_modules/regenerator-runtime/runtime.js:114:21)
    at step (/home/andre/Git/multi-account-containers/node_modules/web-ext/dist/webpack:/node_modules/babel-runtime/helpers/asyncToGenerator.js:17:1)
    at /home/andre/Git/multi-account-containers/node_modules/web-ext/dist/webpack:/node_modules/babel-runtime/helpers/asyncToGenerator.js:30:1
    at processTicksAndRejections (node:internal/process/task_queues:93:5)

I'm a JS noob so please bear with me if I ran into an obvious issue.

Expected behavior

Being able to build the plugin without an exception.