Hi,
- `You cannot resize this side window.`
We can resize the sidebar window by dragging from the edge of the sidebar. The grabbable edge for resizing is quite thin, so we need to move cursor slowly to see the drag cursor icon. Can you try it? If it's width is limited to some maximum value, can you try userChrome.css with

#sidebar-box {
max-width: none !important;
}

You can search step-by-step guide on how to set userChrome.css in case you're not aware of this.

OR
If you're using old sidebar, can you use new sidebar layout?
Settings > General > Browser Layout > Check 'Show Sidebar'
If this option is not there, you can try: about:config > sidebar.revamp to true

OR
Check whether you're using the latest or recent versions of firefox? Sometimes this can also be reasons for compatibility issues.

These should fix the fixed width of sidebar.

Thank you for using this extension.