First you should make sure you have an up to date graphics driver
- old drivers can occasionally cause seams (lines like you show).
Secondly make sure 'Sampling' is set to 'Point' and 'Pixel rounding' is 'On' in project properties. That prevents images being shown at sub-pixel offsets which can cause seams.
One more thing is switching the WebGL mode might make a difference, but it's not always a good fix.
If there is still a problem then, it's probably a browser bug. If this happens please send me your .capx and let me know which browsers you're having a problem with. I can make bug reports to the browser makers if necessary.