RAW Fireball appears to ignore full cover, if there is a path fully within range of the effect that can reach the target, as this spell specifically travels around corners.
If there are no corners this spell can successively travel around within it's effect radius to reach the target, then the target cannot be reached by the effect, and has total cover, causing it to ignore the effect entirely.
Note, there is no limit specified by the spell, stating how many corners can be traveled around, or how arbitrarily complex the path it must follow can be. This leads me to conclude, if you can draw a line that remains entirely within the effect radius that reaches the target, they are hit by the spell.
Also, another thing to note, this is not a point originated explosion as in normal physics, but rather a magical effect that fills any area it can reach with magical flames.
First we need to determine the Grease spells area:
Slick grease covers the ground in a 10-foot square centered on a point within range
Then we need to determine what "enters spell area" means. I did not find clear definition, but if you fly over the area of Grease spell, it'd take a special kind of DM to consider that "entering".
Looking at other spells, Thunderwave clearly specifies an area that is a cube. So I think the normal English meaning of Grease spell's area is quite clear: there is a thin, almost 2-dimensional square layer of slipperiness covering the ground, and to enter it you have to basically touch the ground. If the area of effect had a volume/height, the spell would say so, talking about cube or cylinder.
So, if your feet do not even touch the ground covered by the Grease spell effect, you are not entering it. If you jump completely over it, you won't touch it. So jumping far enough avoids the spell.
Then about STR 10: Strict RAW, with STR 10, you cover only 10', not 10' + one step. If you keep just jumping, you advance 10' per jump, which includes the space taken by your feet. If you kept jumping, you would move forward in 10' increments (well, less for standing jump, but the same principle applies).
When you make a long jump, you cover a number of feet up to your Strength score if you move at least 10 feet on foot immediately before the jump.
So you'd have to either jump from or land at the greasy area, and while you might avoid the difficult terrain (because you don't actually spend movement walking in the grease), you'd at least need to make a DEX saving throw. With this reading, you'd need 1 (or even 2, one at both sides) extra foot, in other words STR 11 (or 12), to avoid the grease completely by jumping.
RAI, hard to say. The rule could be interpreted meaning, you can jump past an obstacle of that many feet, and a DM could reasonably rule it that way. The argument for this interpretation is, that the rules don't say how many extra feet of jump distance you need to get over an obstacle like this, so the reasonable ruling is, that no extra feet are needed.
Best Answer
From Players Handbook, Spellcasting, subsection "Targets":
When you cast an area spell, you can place its point of origin on your side of a wall, but not on the opposite side of the wall.
From Players Handbook, Spellcasting, subsection "Areas of Effect":
Lets say you cast Spike Growth and put its point of origin near a wall, at a location that isn't behind total cover. The spell's radius would extend in all directions from its point of origin, but it would stop at the wall. The ground behind the wall would be unaffected.
This is the cast for most spells. However, there are exceptions (such as Fireball) that specifically mention going around corners. In those cases, the spell's text overrides the general rule for other spells, and so that spell's effects will not necessarily stop at a corner.