Operating system defaults and most, if not all, browsers use Ctrl = as the default zoom in shortcut.
OS defaults can be changed so that Ctrl = is no longer the default zoom, but the browsers (Firefox, Safari) override the system defaults with their default Ctrl = shortcut for zoom in. Furthermore, there is not a straightforward and easy way to change the default shortcuts for zoom in Firefox or Safari browsers.
This means auto completion in Zeppelin is currently unusable. (which is very sad )
Please change to tab completion, for Spark, PySpark, and Python in Zeppelin.
Python in Jupyter notebook (also browser-based) uses tab completion. Tab completion doesn't usually interfere with using the tab key in Python because a tab isn't normally required after a period such as "df." when one would be using the tab completion.