Startpage

Dracula theme (forked): Kanagawa theme (custom made):

When I want to switch themes, I simply rename the HTML file I want to be my current theme to index.html and give the others different names, since Firefox is setup to always open index.html file as startup page.

Settings

Press s to open settings.

Firefox

Setting the startpage as the Home page

1. Copy the startpage dir to ~/.mozilla/firefox/PROFILE/ (You can find your profile by going to about:profiles)
2. Open index.html in Firefox and copy the link in address bar. It would loook like: file:///home/<username>/.mozilla/firefox/<PROFILE>/startpage/index.html
3. Open preferences > Home from the hamburger menu, select custom URLs and paste the address
4. Restart Firefox

Setting the startpage as the New Tab page

1. Write the following lines to /usr/lib/firefox/mozilla.cfg (Create it if it doesn't exist). And change the var newTabUrl = ....

//
var { classes: Cc, interfaces: Ci, utils: Cu } = Components;

/* set new tab page */
try {
  Cu.import("resource:///modules/AboutNewTab.jsm");
  var newTabURL = "file:///home/<username>/.mozilla/firefox/<profile>/startpage/index.html";
  AboutNewTab.newTabURL = newTabURL;
} catch (e) {
  Cu.reportError(e);
} // report errors in the Browser Console

2. Write the following lines to /usr/lib/firefox/defaults/pref/local-settings.js:

//
pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0);
pref("general.config.sandbox_enabled", false);

3. Restart Firefox