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If you use Firefox and Roboform together, and find the webmail interface slow, the problem may be Roboform.
The main symptom of the problem is that page loads are very slow. When clicking on a link or button, the Firefox tab will change to “Loading” with the spinning loading icon, and the page will appear to load after a short moment, but then the browser will completely “freeze” (no spinning loading icon, no scrolling or interaction with any element on the page possible, CPU pegged to 100%) for 1-10 seconds before finally becoming interactive again. The amount of time the freeze occurs for is often proportional to the size of the page, so displaying 500 rows of a mailbox will take many seconds, even on a fast machine.
After some testing and disabling of Firefox extensions one by one, I tracked the problem down to Roboform (the problem occurs even with the as current latest version 6.9.98).
So if you are also experiencing this problem, as a first step, you can use Tools –> Add-ons to disable the Roboform extension inside Firefox. After disabling the extension and then restarting Firefox, you should find the web interface a lot faster.
As a long term solution, I recommend using the password manager built into Firefox instead, and completely removing Roboform. Unfortunately I don’t know an easy way of transferring passwords from Roboform to Firefox, so I’ve had to do it manually one by one. Within Firefox, you can also use the free extension SyncPlaces to store a copy of your Firefox passwords and bookmarks in your FastMail file storage area via WebDAV. You can also use SyncPlaces on multiple machines to keep your bookmarks and passwords in sync across those machines.