Bookmark menu freezes up when I try to delete or transfer bookmarks to another folder

Question

Last March, I transited from a Mac-based XP to a new desktop with Win7. FF was working fine and I was backing up my bookmarks with Xmarks/Sync.
I used XMarks/Sync for the transfer of my bookmarks in the new machine and I upgraded from FF9 to 11. I did not create any new add-ons.
All appeared normal except that in upgrading to 12 soon after, I found that I lost the Bookmarks organizer. This was a handicap as I used it frequently to tidy up my bookmark folders. I was forced to resort to a click-and-drag method to achieve the same result and this is when I discovered that this was “freezing” the bookmarks menu list (on the LHS of FF page display). Once I had moved or deleted one item I was unable to do it again. I tried to release the blockage by moving the cursor over the page or over the header bar, it seemed to work once or twice then no more.
I had to wait several minutes between each bookmark transfer or deletion. I found that by logging off & on FF, I could remove the waiting but doing that non-stop on a large scale started causing Internet problems and besides, the unfreezing of the bookmarks stopped working with that also eventually.
I really do not know what to do.
Uninstalling and re-installing FF12 did not help. I only have 4 basic add-ons like dictionary, translator and Pop-up blocker. I use AVG "do-not-tack".
It seems to me that whenever I make a change to my bookmark list, FF is “waiting” for something else to complete it (i.e.: Foxmark/Sync?) and it is that unknown feature which causes the lengthy hanging. I am probably wrong but I really do not know where to turn for help at this point.
Anyone PLEASE? Thank you in advance.

Answer 1

Thank you cor-el for your comprehensive response, I have began to follow through each step sequentially and the first link already disabled half of my add-ons, which is fine with me. I do not think I will miss them.
Since FF works fine except for the bookmarks freezing, I found one option which seems to target that perfectly so I will proceed with this one first and hope that it will take care of the problem.
Great support work, thanks again,
Arlette