Another way is tweaking the CSS directly (e.g. with the "Stylus" extension/addon, available for Chrome and Firefox).
This code snippet can be imported right away into "Stylus":
Code:
@-moz-document url-prefix("https://www.fastmail.com/mail/") {
/* Suppress "+/Edit/Find"-Toolbar */
[class^="v-Sidebar-toolbar"] {
display:none !important;
}
/* Suppress Quota Box */
[class^="v-Quota"] {
display:none !important;
}
}