Are you using Text Zoom or Minimum Font Size? These features require quite a lot of processing, especially on some complex pages. You can avoid most of this processing by disabling the "Apply to dynamic content (better but slower)" option.
The Alt+Z shortcut should always work. The other shortcuts (Ctrl+Plus, Ctrl+Minus, Ctrl+0, Ctrl+9, Ctrl+8, Ctrl+7) only work if the page contents are focused, otherwise the first three (Ctrl+Plus, Ctrl+Minus, Ctrl+0) invoke Firefox's standard zoom shortcut actions.
There is a button in Zoom Page WE Options to clear all per-site data. The reasoning is that this button will not be used very often and having it in the options avoids clutter in the zoom popup.
Right-click toolbar button > Manage Extension does work. It shows a description of Zoom Page WE and an Options button at the bottom of the page.
There is no online issues page. All problem reports should be emailed to: firstname.lastname@example.org