For Month and Multiweek views, the calendar background is just one color. The only thing visually separating one month from another are the day numbers and a small 3 letter Month abbreviation on the 1st box of every month. When scrolling through the month it is visually difficult to know when to stop as the months just go by all one color with no visual delineation.
I have seen other calendar software and apps deal with this in various ways :
1- A thicker lined perimeter border around each month.
2- Each alternate month having a slight background shade. Example: white/grey/white etc.
3- Overlaying a large 3 letter Month abbreviation over the center of each month.
I have included 2 screen shots of one calendar showing alternate month shading, and another showing month outline as well as 3 letter month overlay which scroll with the months.
In Thunderbird, these changes should apply to both MONTH VIEW and MULTIWEEK VIEW. Both of which are essentially scrolling through months.