Fixed
The popup width setting works again on Firefox. A filled popup opened nearly twice as wide as an empty one, and neither Popup width nor Expanded popup width had any effect.
The background page passes the options to the popup over runtime.sendMessage, which clones them. Three options — the update interval and the two widths — are computed rather than stored directly, and Firefox's clone drops computed properties. The popup received no width at all, and since it sizes itself to its contents, it grew to fit the longest article title instead of the width you configured. An empty popup had nothing to grow around, which is why only a filled one looked wrong.
The widths were being lost on Firefox before 3.3.0 as well, but the old article layout happened to hide it. The layout change in 3.3.0 did not cause the bug, it revealed it. Chromium was unaffected throughout.
Both widths are now sent as plain values, and the popup falls back to its 380px minimum rather than silently applying nothing.